KABUL —

The U.S. military is establishing "alternative routes" to Kabul airport because of a threat the terror group ISIS-K poses to the airport and its surroundings, as President Joe Biden met with senior officials Saturday to discuss the security situation in Afghanistan and counter-terrorism operations against the Islamic State offshoot.

"There is a strong possibility ISIS-K is trying to carry off an attack at the airport," a U.S. defense official told CNN. A senior diplomat in Kabul said they are aware of a credible but not immediate threat by Islamic State against Americans at Hamid Karzai International Airport.

Two U.S. defense officials described the military effort to establish "alternative routes" for people to get to Kabul airport and its access gates, with one saying these new routes will be available to Americans, third party nationals and qualified Afghans.

The Taliban are aware of the new effort and are coordinating with the U.S., one of the officials said.

Possible threats

The Pentagon has been monitoring the situation around the airport, aware that the swelling crowds on the grounds and around the airfield create a target for ISIS-K and other organizations, which may use car bombs or suicide bombers to attack, the second official said. Mortar attacks are another possible threat.

Details of the plan are being closely held, but the broadly sketched-out details call for people to follow new routes and access points in coordination with Taliban on the ground in an attempt to help disperse the gathering of large crowds or avoid the crowds altogether, the two officials said. U.S. personnel would be in a position to observe the movement of people to ensure safety, but the official would not specify if that involves direct observation by nearby troops as well as the use of intelligence sensors.

"There's a whole canopy of security concerns we have," Pentagon press secretary John Kirby said at a press briefing Saturday, as he described the military "fighting against both time and space" in its effort to safely evacuate people.

"The idea is to get as many people out as fast as we can," Kirby said. "That's what the focus is. In trying to accomplish that mission, we're taking in a whole wealth of information about what the security environment looks like."

Biden and his national security team met in the Situation Room on Saturday morning to discuss "the security situation in Afghanistan and counterterrorism operations, including ISIS-K," the White House said. "They discussed the massive logistical operation to evacuate American citizens and their families, SIV applicants and their families, and vulnerable Afghans both on U.S. military aircraft, as well as flight charters and coalition flights."

ISIS-K is a self-proclaimed branch of the terror group that first emerged in Syria and Iraq. While the affiliates share an ideology and tactics, the depth of their relationship with regard to organization and command and control has never been entirely established.

US intelligence officials previously told CNN the ISIS-K membership includes "a small number of veteran jihadists from Syria and other foreign terrorist fighters," saying that the US had identified 10 to 15 of their top operatives in Afghanistan. The group's name comes from its terminology for the area that includes Afghanistan and Pakistan: "Khorasan."

Biden referred to the threat from Islamic State in an address to the nation on Friday, telling Americans that, "we're also keeping a close watch on any potential terrorist threat at or around the airport, including from the ISIS affiliates in Afghanistan who were released from prison when the prisons were emptied."

Biden noted that ISIS in Afghanistan has been the sworn enemy of the Taliban, with which U.S. officials have been coordinating and communicating on a constant basis over access to the airport.

"I've said all along," Biden added, "We're going to retain a laser-focus on our counterterrorism mission, working in close coordination with our allies and our partners and all those who have an interest in ensuring stability in the region."

'The best job they can'

According to an official familiar with the matter, Biden has pushed his team to ramp up flights and evacuations but accessing the airport has become difficult as crowds swarm the gates.

Not long before the President met to confer with Vice President Kamala Harris, Defense Secretary Lloyd Austin, Joint Chiefs Chairman Mark Milley, Secretary of State Antony Blinken and others on Saturday, the U.S. embassy in Kabul sent a security alert saying that "because of potential security threats outside the gates at the Kabul airport, we are advising U.S. citizens to avoid traveling to the airport and to avoid airport gates at this time unless you receive individual instructions from a U.S. government representative to do so."

At the Pentagon, where an official told CNN on Saturday that evacuations had slowed in the past 24 hours, spokesman Kirby said the State Department is "doing the best job they can to advise Americans who still haven't made it to the airport, what the situation looks like around the airport, that would be the prudent thing to do."

"If you're an American and you're at a gate, you will be let in that gate," Kirby said Saturday.

But gates to the airport have been closed for "short durations" over the past 24 hours, Gen. Hank Taylor, deputy director of the Joint Staff for Regional Operations, told reporters during the Pentagon briefing with Kirby.

"There have been short durations throughout the last 24 hours where gates have been closed to allow the proper people to come in and out of those gates," Taylor said.

The President asserted on Friday that there's no indication American citizens have been prevented from getting through to the airport but acknowledged the risks involved with the evacuation mission, saying it "is dangerous, involves risks to Armed Forces, and has been conducted under difficult circumstances."

Biden stressed that he could not promise what the final outcome will be, or "that it will be without risk of loss." But he added that "as Commander in Chief, I can assure you that I will mobilize every resource necessary."

Speaking to reporters Saturday, Kirby did not rule out taking other measures to get Americans to Kabul airport, including having U.S. military troops go into the city and retrieve them, if necessary. On Friday, the Pentagon revealed it had used three CH-47 Chinook helicopters to retrieve 169 Americans who had gathered at a hotel about 200 meters from an airport gate, wary of the risks involved in trying to walk through the throngs outside amid reports of violence and Taliban beatings.

"We're going to continue to explore options to assist Americans as needed," Kirby said. "We will do that here at the Pentagon. If there's a need to do something different than what we're already doing to facilitate them getting into the airport ... we'll certainly consider those options."

'Fighting against both time and space'

But Kirby acknowledged the challenge the military is facing as it works towards an August 31 deadline to leave the country. Biden has indicated the U.S. may have to stay beyond that date if all Americans have not been evacuated yet.

"I think we've been very honest about the fact that we know that we're fighting against both time and space," Kirby said. "That's really what, that's the race that we're in right now, and we're trying to do this as quickly and as safely as possible."

The pace of the evacuation effort slowed after a bottleneck developed Friday as space at the Al Udeid Air Base in Qatar, one of the leading destinations for flights, neared capacity, forcing the US to scramble for other locations.

Only 6 C-17s have left Kabul international airport in the past 24 hours, carrying some 1,600 people, a defense official told CNN, a dramatic reduction in the pace of evacuations as a result of Friday's 8-hour delay in flights.

That figure was a stark drop from the 6,000 people who flew out of Kabul in the previous 24-hour period on 16 C-17 flights and a C-130, according to figures Taylor gave reporters during a Pentagon briefing on Friday.

On Saturday, Taylor told reporters that on military aircraft and charter planes combined, approximately 3,800 people were evacuated in the past 24 hours.

Since the end of July, 22,000 people have been evacuated, with 17,000 of them flown out over in the week since August 14, Taylor said. Out of the 17,000 evacuated since August 14, 2,500 are U.S. citizens, Taylor said.

C-17 military planes are now "moving between Qatar and Germany," Taylor said, and in the past 24 hours, three flights from Kabul landed at Dulles International Airport near Washington, DC. Some Afghans will be transitioned to Fort Bliss for further processing, Taylor added.

On Friday, U.S. officials announced a dramatic expansion in the number of countries that will help transit Americans or temporarily host Afghans, including Germany where the first evacuation flight of about 350 people arrived at Ramstein Air Base.

The United States is struggling to pick up the pace of American and Afghan evacuations at Kabul airport, constrained by obstacles ranging from armed Taliban checkpoints to paperwork problems. With an Aug. 31 deadline looming, tens of thousands remained to be airlifted from the chaotic country.

Taliban fighters and their checkpoints ringed the airport — major barriers for Afghans who fear that their past work with Westerners makes them prime targets for retribution. Hundreds of Afghans who lacked any papers or clearance for evacuation also congregated outside the airport, adding to the chaos that has prevented even some Afghans who do have papers and promises of flights from getting through.

It didn't help that many of the Taliban fighters could not read the documents.

In a hopeful sign, State Department spokesman Ned Price said in Washington that 6,000 people were cleared for evacuation Thursday and were expected to board military flights in coming hours. That would mark a major increase from recent days. About 2,000 passengers were flown out on each of the past two days, Pentagon spokesman John Kirby said.

Kirby said the military has aircraft available to evacuate 5,000 to 9,000 people per day, but until Thursday far fewer designated evacuees had been able to reach, and then enter, the airport.

Kirby told reporters the limiting factor has been available evacuees, not aircraft. He said efforts were underway to speed processing, including adding State Department consular officers to verify paperwork of Americans and Afghans who managed to get to the airport. Additional entry gates had been opened, he said.

And yet, at the current rate it would be difficult for the U.S. to evacuate all of the Americans and Afghans who are qualified for and seeking evacuation by Aug. 31. President Joe Biden said Wednesday he would ensure no American was left behind, even if that meant staying beyond August, an arbitrary deadline that he set weeks before the Taliban climaxed a stunning military victory by taking Kabul last weekend. It was not clear if Biden might consider extending the deadline for evacuees who aren't American citizens. The president will deliver remarks on the evacuation Friday afternoon at the White House.

At the Kabul airport, military evacuation flights continued, but access remained difficult for many. On Thursday, Taliban militants fired into the air to try to control the crowds gathered at the airport's blast walls. Men, women and children fled. U.S. Navy fighter jets flew overhead, a standard military precaution but also a reminder to the Taliban that the U.S. has firepower to respond to a combat crisis.

There is no accurate figure of the number of people — Americans, Afghans or others — who are in need of evacuation as the process is almost entirely self-selecting. For example, the State Department says that when it ordered its nonessential embassy staff to leave Kabul in April after Biden's withdrawal announcement, fewer than 4,000 Americans had registered for security updates. The actual number, including dual U.S.-Afghan citizens along with family members, is likely much higher, with estimates ranging from 11,000 to 15,000. Tens of thousands of Afghans may also be in need of escape.

Compounding the uncertainty, the U.S. government has no way to track how many registered Americans may have left Afghanistan already. Some may have returned to the United States but others may have gone to third countries.

At the Pentagon, Kirby declined to say whether Defense Secretary Lloyd Austin had recommended to Biden that he extend the Aug. 31 deadline. Given the Taliban's takeover of the country, staying beyond that date would require at least the Taliban's acquiescence, he said. He said he knew of no such talks yet between U.S. and Taliban commanders, who have been in regular touch for days to limit conflict at the airport as part of what the White House has termed a "safe passage" agreement worked out on Sunday.

"I think it is just a fundamental fact of the reality of where we are, that communications and a certain measure of agreement with the Taliban on what we're trying to accomplish has to occur," Kirby said.

Of the approximately 2,000 people airlifted from the airport in the 24 hours ended Wednesday morning, nearly 300 were Americans, Kirby said. U.S. lawmakers were briefed Thursday morning that 6,741 people had been evacuated since Aug. 14, including 1,762 American citizens and Green Card holders, according to two congressional aides.

Although Afghanistan had been a hotspot for the coronavirus pandemic, the State Department said Thursday that evacuees are not required to get negative COVID-19 results.

"A blanket humanitarian waiver has been implemented for COVID-19 testing for all persons the U.S. government is relocating from Afghanistan," the department said. Medical exams, including COVID-19 tests, had been required for evacuees prior to the Taliban's takeover of Kabul, which added extra urgency to efforts to get at-risk Afghans out.

Additional American troops continued to arrive at the airport. As of Thursday there were about 5,200, including Marines who specialize in evacuation coordination and an Air Force unit that specializes in emergency airport operations. Biden has authorized a total deployment of about 6,000.

Hoping to secure evacuation seats are American citizens and other foreigners, Afghan allies of the Western forces, and women, journalists, activists and others most at risk from the fundamentalist Taliban.

In June, more than 20 diplomats at the U.S. Embassy in Kabul registered their concerns that the evacuation of Afghans who had worked for America was not proceeding quickly enough.

In a cable sent through the State Department's dissent channel, a time-honored method for foreign service officers to register opposition to administration policies, the diplomats said the situation on the ground was dire, that the Taliban would likely seize control of the capital within months of the Aug. 31 pullout, and urged the administration to immediately begin a concerted evacuation effort, according to officials familiar with the document.

Will U.S. troops go beyond the airport perimeter to collect and escort people? Austin suggested on Wednesday that this was not currently feasible. "We don't have the capability to go out and collect large numbers of people," he told reporters.

Austin added that evacuations would continue "until the clock runs out or we run out of capability."

Afghans in danger because of their work with the U.S. military or U.S. organizations, and Americans scrambling to get them out, also pleaded with Washington to cut the red tape that has complicated matters.

"If we don't sort this out, we'll literally be condemning people to death," said Marina Kielpinski LeGree, the American head of a nonprofit, Ascend. The organization's young Afghan female colleagues were in the mass of people waiting for flights at the airport in the wake of days of mayhem, tear gas and gunshots.

The Pentagon says flights have resumed from an airport in Kabul, Afghanistan, a day after video footage showed hundreds of Afghans rushing the runway in an attempt to board U.S. planes and leave the country.

In multiple media appearances Tuesday morning, Pentagon spokesperson John Kirby said that flights have resumed, and the Defense Department is working to move both Americans and Afghans out of the country.

"The airport is up and running; operations are continuing," Kirby told ABC News.

Kirby noted that the U.S. moved between 700 and 800 people out of Afghanistan in the last 24 hours, including 150 American citizens and many Afghan citizens. During that same period, an additional 1,000 American troops arrived in Kabul, with more to come in the days ahead.

During a press briefing on Monday, joint staff director of current operations Army Maj. Gen. Hank Taylor said that the Department of Defense's focus remained on protecting the airport. By his estimation, the military could evacuate between 5,000 and 9,000 people from the country each day.

During Monday's briefing, Kirby also reiterated that the evacuation mission would end, at the White House's direction, on Aug. 31. That means as many as 135,000 people could be flown out of the country in the next two weeks.

It's unclear how many U.S. citizens, Afghani special immigrant visas and third-party foreign nationals still need to be evacuated.

Flights were suspended from the airport on Monday after hundreds of people rushed to the airport in an attempt to flee the country from Taliban rule. The stunning footage showed crowds of people abandoning their cars on local highways, jumping fences and running right onto the tarmac.

An additional video showed people running beside a U.S. Air Force plane as it prepared to take off. Some clung to the aircraft's fuselage as it lifted into the air.

The Associated Press reported Monday that seven people died in the chaos at the airport.

Amid the disarray, the State Department advised Americans in the area to shelter in place. On Monday, Kirby told media outlets that the Department had reached a message to those sheltering and had "given them cues" as to how to assemble at the airport for evacuation.

Kirby added that the Pentagon is planning to house up to 22,000 Afghan citizens at three U.S. installations once they are evacuated from the country, many of them translators or those who have assisted American troops over the years.

"It's not just about moving out Americans; it's very much about meeting our moral and sacred obligations to those Afghans who helped us over the last 20 years, getting as many of them out as we can," Kirby said.

During the White House press briefing on Tuesday, national security adviser Jake Sullivan again defended the Biden administration's continued troop drawdown. While he admitted there were "chaotic scenes" at the airport on Monday, he said the Pentagon had made appropriate adjustments to keep the area secured in the hopes of transporting as many people out of the country as possible in the coming days.

"The images at the airport the past couple of days have been heartbreaking. But President Biden had to think of the alternative path as well, which was to stay in the middle of a civil conflict in Afghanistan," Sullivan said. "There are those who argue that with 2,500 forces, the number of forces in Afghanistan when President Biden took office, we could have sustained a peaceful Afghanistan. That is simply wrong."

Psaki later said that Biden continues to have faith in his intelligence advisers and that he and the intelligence agencies remain in "lockstep."