On Aug. 6, 1945, the United States dropped an atomic bomb on the Japanese city of Hiroshima — becoming the only country to ever use nuclear weapons during wartime.Watch the video above to learn morePresident Harry S. Truman made the decision to use the atomic bomb against Japan in hopes that it would speed up the end of World War II, and also as retaliation for their attack on the U.S. naval base at Pearl Harbor, which killed more than 2,400 Americans."The Japanese began the war from the air at Pearl Harbor. They have been repaid manyfold. And the end is not yet," Truman wrote in a statement announcing the nuclear attack.Video below: President Truman announces atomic bomb dropped on HiroshimaThe a-bomb blast, equivalent to the power of 15,000 tons of TNT, instantly killed 80,000 people and injured another 35,000. At least 60,000 more died in the years to follow from radiation poisoning they suffered as a result. The U.S. began working on developing the atomic bomb in 1940 after being made aware that Nazi Germany was researching how to create nuclear weapons. The U.S. launched its first successful a-bomb test in the desert in New Mexico in July of 1945 — and although Germany had already been defeated, the war was still raging against Japan.Three days after the bombing of Hiroshima, on Aug. 9, 1945, the U.S. unleashed another atomic bomb on Japan on the city of Nagasaki — instantly killing nearly 40,000 more people.Less than a week later, Japan announced its surrender on Aug. 14, 1945. Japan formally surrendered in writing on Sept. 2, 1945, officially ending World War II.

Europe's largest nuclear power station is on fire following an attack by Russian troops.

U.S. President Joe Biden spoke with President Volodymyr Zelenskyy of Ukraine and has also urged Russia to stop military activities in the area of the Zaporizhzhia Nuclear Power Plant so that firefighters and emergency responders can access the site and put out the flames.

President Biden has also been in touch with the Under Secretary for Nuclear Security of the U.S. Department of Energy and Administrator of the National Nuclear Security Administration to be updated on the situation at the nuclear plant.

Ukraine's foreign minister has called for a security zone, and also pressed the importance of immediately allowing firefighters to be allowed to go in and handle the blaze, Reuters reported.

The fire at the nuclear power station in the city of Enerhodar comes after Russian troops were shelling it just before, Ukraine's foreign minister said.

“We demand that they stop the heavy weapons fire,” Andriy Tuz, spokesperson for the plant in Enerhodar, said in a video posted on Telegram. “There is a real threat of nuclear danger in the biggest atomic energy station in Europe.”

Ukraine's Foreign Affairs Minister Dmytro Kuleba tweeted late on Thursday, "Fire has already broke out. If it blows up, it will be 10 times larger than Chornobyl! Russians must IMMEDIATELY cease the fire, allow firefighters, establish a security zone!"

The plant accounts for about one quarter of Ukraine’s power generation.

U.S. Secretary of Energy Jennifer Granholm said she has spoken with Ukraine's energy minister about this situation and called on Russian troops to cease their attack calling the situation "reckless." She said the U.S. Department of Energy has activated its Nuclear Incident Response Team and is monitoring the situation along with the U.S. Department of Defense and the U.S. Nuclear Regulatory Commission.

The fighting at Enerhodar, a city on the Dnieper River, came as another round of talks between the two sides yielded a tentative agreement to set up safe corridors inside Ukraine to evacuate citizens and deliver humanitarian aid.

The mayor of Enerhodar said Ukrainian forces were battling Russian troops on the city’s outskirts. Video showed flames and black smoke rising above the city of more than 50,000, with people streaming past wrecked cars, just a day after the U.N. atomic watchdog agency expressed grave concern that the fighting could cause accidental damage to Ukraine’s 15 nuclear reactors.

Earlier, both the Ukrainian state atomic energy company and the mayor of Enerhodar, Dmytro Orlov, said Russian troops were approaching the Zaporizhzhia nuclear plant. Officials said loud shots were heard in the city late Thursday.

“Many young men in athletic clothes and armed with Kalashnikov have come into the city. They are breaking down door and trying to get into the apartments of local residents,” the statement from Energoatom said.

Ukrainian Prime Minister Denys Shmyhal has joined Ukraine’s president in calling on the West to close the skies over Ukraine’s nuclear plants as fighting intensified around the major energy hub on the left bank of the Dnieper River and the Khakhovka Reservoir.

Shmyhal said he already had appealed to NATO and the International Atomic Energy Agency, the United Nations' atomic watchdog.

“Close the skies over Ukraine! It is a question of the security of the whole world!” Shmyhal said in a statement Thursday evening.

The U.S. and NATO allies have ruled out creating a no-fly zone since the move would directly pit Russian and Western militaries.

BERLIN — The United Nations’ atomic watchdog says Ukraine has informed the International Atomic Energy Agency that staff who have been kept at the Chernobyl Nuclear Power Plant since Russian troops took control of the site a week ago are facing “psychological pressure and moral exhaustion.”

IAEA Director General Rafael Mariano Grossi said Thursday that the staff must be allowed to rest and rotate so their crucial work can be carried out safely and securely.

Grossi received “a joint appeal from the Ukraine Government, regulatory authority and the national operator which added that personnel at the Chornobyl site ‘have limited opportunities to communicate, move and carry out full-fledged maintenance and repair work,’” the IAEA said in a statement.

Reactor No. 4 at the power plant exploded and caught fire in 1986, shattering the building and spewing radioactive material high into the sky. Even 36 years later, radioactivity is still leaking from history’s worst nuclear disaster.

Ukraine has lost regulatory control over all the facilities in the Chernobyl Exclusion Zone to the Russians and asked the IAEA to undertake measures “in order to reestablish legal regulation of safety of nuclear facilities and installations” within the site, the statement added.

Grossi has repeatedly stressed that any military or other action that could threaten the safety or security of Ukraine’s nuclear power plants must be avoided.

“I remain gravely concerned about the deteriorating situation in Ukraine, especially about the country’s nuclear power plants, which must be able to continue operating without any safety or security threats,” he said. “Any accident caused as a result of the military conflict could have extremely serious consequences for people and the environment, in Ukraine and beyond.”

The U.S. and its allies restart Iran nuclear talks on Monday unsure how Tehran's new government will approach negotiations, not optimistic about the prospects ahead and emphasizing that if diplomacy fails, the U.S. is "prepared to use other options."

The parties to the Joint Comprehensive Plan of Action will reconvene in Vienna after almost six months to discuss a mutual return to the deal by both the U.S. and Iran, but the hiatus has given time for new obstacles to take root.

Video above: Trump's Iran deal exit 'very bad judgments,' Biden said earlier this month

On Friday, Iran announced yet more advances in its uranium enrichment, which reduces the amount of time Tehran would need to develop a nuclear weapon, if it chooses to, an announcement clearly meant to give Iran leverage when it arrives in Vienna for talks.

Other parties to the agreement -- including Germany, the UK, Britain, France, China and Russia -- are coming into the talks calling for negotiations to pick up where they left off. European sources tell CNN they expect the Iranians to treat the meeting as "round one." U.S. officials have expressed similar concerns.

The recently elected hardline government in Tehran will send a new set of negotiators to Vienna who have been emphasizing the need for complete U.S. sanctions relief, not compliance with the deal, while U.S. officials have said they have absolutely no plans to offer Iran incentives to talk.

'The time to choose is short'

And senior U.S. officials have repeatedly warned that if advances in Iran's nuclear program and enrichment capability continue unabated, they could render the benefits of the JCPOA moot -- a development that would force the U.S. to pursue other options.

"We are still hopeful that diplomacy can find a way," Brett McGurk, the National Security Council's coordinator for the Middle East and North Africa, told the Manama Dialogue organized by the International Institute for Strategic Studies. "But if it cannot find a way, we are prepared to use other options."

"There is no question, we are not going to allow Iran to get a nuclear weapon, period," McGurk said. "And when it came to military force for behavior change, that is a pretty fuzzy objective for a military force. When it comes to military force to prevent a country from obtaining a nuclear weapon, that is a very achievable objective."

U.S. Special Envoy for Iran Rob Malley said in a tweet after a Nov. 18 meeting with Middle Eastern allies and European parties to the deal that Iran could choose one of two paths: "continued nuclear escalation & crisis, or mutual return to the JCPOA, creating opportunities for regional economic & diplomatic ties."

"Time to choose is short," Malley wrote.

Sources familiar with preparations for the talks say that the parties were closely watching International Atomic Energy Agency director Rafael Grossi's visit to Tehran last week, seeing it as an indication of Iran's approach to the talks in Vienna, those sources said. Grossi told the IAEA board afterward that the talks were "inconclusive."

One of the contentious issues remaining is that Iran is refusing inspectors from the IAEA monitoring access to the Karaj centrifuge production facility, which reports suggest has resumed operations.

"This is seriously affecting the [IAEA's] ability to restore continuity of knowledge at the [Karaj] workshop, which has been widely recognized as essential in relation to a return to the JCPOA," Grossi told a Board of Governors meeting on Wednesday.

The Arms Control Association noted that Iran's refusal to allow the IAEA to install new cameras or confirm that production hasn't restarted could undermine attempts to revitalize the JCPOA and its strict verification regime if it isn't possible to fully complete records of Iran's nuclear program. Tehran's refusal to grant access to Karaj also drives speculation and concern about what, exactly, Iran is doing, the ACA said.

'No choice'

On Thursday, the U.S. Mission to International Organizations in Vienna told the IAEA meeting that "if Iran's non-cooperation is not immediately remedied ... especially the restoration of continuity of knowledge at Karaj, the Board will have no choice but to reconvene in extraordinary session before the end of this year in order to address the crisis."

Meanwhile, on Friday, Iran announced its stock of 60% enriched uranium has grown to 66 pounds and its amount of 20% enriched uranium had also increased. Both levels are much closer to weapons-grade uranium which is enriched above 90%.

According to the Arms Control Association, enriching uranium to 20% "constitutes about 90 percent of the necessary work to enrich to weapons-grade."

As Iran's stockpiles grow, the ACA says, its breakout time, or the time it would take to produce enough uranium enriched to weapons-grade for one bomb, decreases. The ACA estimates that Iran's current breakout time is likely about one month, down from 12 months when the JCPOA was fully implemented.

Enrichment was limited under the JCPOA, which the U.S. left unilaterally in May 2018 under former President Donald Trump. Iran restarted enrichment last year to pressure the U.S. to ease sanctions.

'A very uncertain proposition'

State Department spokesman Ned Price reflected the ambiguity surrounding the resumed talks on Nov. 22, calling the mutual return to compliance with the JCPOA "a very uncertain proposition."

The next day, Price told reporters in Washington that, "it is our hope that the new government in Iran shows up in Vienna and shows up in Vienna ready to negotiate in good faith to build on the progress that had been achieved in the previous six rounds of negotiations."

But he added that the U.S. has "been very clear that we are not prepared to take unilateral steps solely for the benefit of greasing the wheel" to get the talks going again. Former President Donald Trump pulled the U.S. from the deal in 2018.

Sources familiar with the preparations for the talks have told CNN that the U.S. and its allies are not at a point where they would begin offering Iran confidence-building measures, but one official said there is a possibility the U.S. and its allies could employ them down the road. As a result, incentives for Iran won't be discussed at this week's meetings in Vienna, where the U.S. and allies will be focused on simply taking the temperature and seeking to advance from where they left off months ago, U.S. and European sources explained.

'Plan B'

Everyone involved in the talks is mindful of the ticking clock. The sources told CNN that there's still time to reach a deal, but it would likely run out by the end of next year. For now, they said there is no hard and fast "Plan B" yet.

Critics of the deal say that the Biden administration has sacrificed leverage by easing pressure on Iran while it builds up its nuclear program.

"The Biden administration's Iran policy is failing, and without a significant course correction that policy will either result in Iranian nuclear weapons or in a war to stop that development," said Mark Dubowitz, the CEO of Foundation for Defense of Democracies. Dubowitz argued that the administration's approach will allow Iran to rebuild toward a "lethal end state" of with pathways to nuclear weapons and a robust nuclear infrastructure.

"Israel is going to have no choice but to use military force to stop Iran's nuclear weapons before Tehran reaches this lethal end state," Dubowitz said.

Israeli Prime Minister Naftali Bennett has been making clear that Israel will be prepared to act if necessary. Addressing delegates at a security conference near Tel Aviv on Tuesday, Bennett said that "if there is a return to the JCPOA, Israel obviously is not a party to the agreement and is not obligated by it."

Bennett complained that after the nuclear deal was signed in 2015, the "State of Israel simply went to sleep. We were occupied with other things. We will learn from this mistake. We will maintain our freedom of action," he said.

Western officials have tried to argue to the Israelis that attacks on Israel's nuclear program are not very useful when the overall goal is to come up with a comprehensive solution, and especially when the Iranians have sped up their capability to rebuild after attacks, sources familiar with the Iran talks have told CNN.

Western officials have also raised the danger of Iran responding with kinetic action, but sources familiar with the talks say Israeli officials still seem to think that it is still an effective tool to show their capabilities.

Asked about those warnings, Price said that, "at the end of the day, the United States and Israel, we share a common objective here, and that is to see to it that Iran is verifiably and permanently prevented from obtaining a nuclear weapon. And we continue to believe that diplomacy in coordination with our allies and partners -- and that, of course, includes Israel -- is the best path to achieve that goal."

"We've also been very clear that this is not a process that can go on indefinitely and if the Iranians through their actions or through their inactions demonstrate or suggest that they lack that good faith, that they lack that clarity of purpose, we'll have to turn to other means," Price said Tuesday. "We have a variety of other means we're discussing those with our allies and partners."

Diplomatic flurry

In recent weeks, U.S. officials have conducted a flurry of diplomacy with regional powers and other parties to the deal, working to forge a united front.

President Joe Biden met with European partners to discuss Iran during the June G7 meetings in the UK. In recent weeks, Secretary of State Antony Blinken has also conferred with European allies, as well as China and Russia, on Iran. And Malley recently met with Gulf countries, Israeli officials and European partners in the JCPOA.

"I think the Iranians believe they have some eastward option with Russia and China in which they can circumvent the pressure of sanctions," McGurk said on Sunday. "And that is just wrong. And so I think we are approaching the talks at the end of November as a pretty united front with the P5+1."