CNN —

North Korea launched two short-range ballistic missiles into the waters off the east coast of the Korean Peninsula on Monday morning, South Korean authorities said, the latest in series of missile firings as South Korean and United States forces engage in their largest military exercises since 2017.

Japanese authorities said the two missiles are suspected of having flown on "an irregular trajectory," with a maximum altitude of 50 kilometers (31 miles) and at a distance of of 350 kilometers (217 miles).

Irregular trajectories could mean the missiles are maneuverable, in contrast to regular ballistic missiles, which travel in an arc.

Video above: State media in North Korea recently claimed its military has tested a nuclear underwater drone that could release a 'radioactive tsunami'

The missile launches come as South Korean and U.S. forces engage in large-scale military exercises involving thousands of troops on the southern part of the Korean Peninsula.

Following Monday's missile launch, the South Korean military said it would continue operating the Warrior Shield field training exercise at high intensity until they end in early April.

Two large U.S. Navy platforms, the aircraft carrier USS Nimitz and the amphibious assault ship USS Makin Island are near the southern coast of South Korea preparing to participate in exercises later this week.

North Korean missile launches tend to spike when Washington and Seoul hold joint military drills.

The latest U.S.-South Korea exercises are among the largest the two allies have put on since they scaled back such military displays in 2017, when then-U.S. President Donald Trump tried to offer an opening for North Korea to negotiate an end to its long-range missile and nuclear weapons programs.

That opening has long since closed, with North Korea last year conducting a record number of missile tests while pledging to develop its nuclear program to arm the missiles.

The North's missile testing has picked up again in recent weeks, as tensions on the Korean Peninsula continue to escalate.

SEOUL, Seoul —

North Korea's recent sword-rattling after months of relative quiet makes clear that leader Kim Jong Un is working on expanding his weapons arsenal.

Nuclear-capable missiles hidden in trains that can be launched anywhere along a railway. A new cruise missile resembling the U.S. Tomahawk that can be potentially topped with atomic warheads. The apparent resumption of making fuel for potential nuclear bombs.

Likely they are an attempt to wrest concessions from Washington if, and when, long-stalled diplomatic talks on Kim's nuclear program resume. Part of the message is aimed domestically, however, to reinforce internal unity as North Koreans cope with deeper hardship in a never-healthy economy that's been battered by the coronavirus pandemic.

Here, then, is a look at Kim's recent weapons tests, the first of their kind in six months, and what they may mean for efforts to confront the North's nuclear ambitions.

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THE NEW WEAPONS

North Korea called its first train-launched ballistic missile tests successful, saying the two weapons launched Wednesday struck a target in the sea 800 kilometers (500 miles) away.

That puts all of South Korea and the U.S. military bases it hosts in range. Experts say the missiles are nuclear-capable.

Firing from trains also adds another platform for missile launches — in addition to mobile trucks, ground pads and a submarine method still being tested. A train-based platform utilizes North Korea's national rail network and allows for secret movement and launch, although experts note rail networks are vulnerable targets in a crisis.

For South Korea, "which has to defend itself from North Korean missiles, it's yet another headache," said Lee Choon Geun, a missile expert at South Korea's Science and Technology Policy Institute.

Last weekend, North Korea also tested what it called a new cruise missile, which flew about 1,500 kilometers (930 miles), making it the North's longest-range cruise missile, capable of reaching all of Japan, which is also home to 50,000 U.S. troops.

It's being developed as nuclear capable and flies at a low altitude, making it harder to detect. Its development demonstrates North Korea's push to break enemy defense lines and diversify a weapons inventory that's heavy on ballistic missiles.

Satellite photos also show signs North Korea has restarted operations at its main factory for producing weapons-grade plutonium, a key ingredient used to make nuclear weapons.

WHAT KIM WANTS

Kim's resumed testing activities are largely meant "for developing military capabilities, but can also be attempts at shoring up domestic unity," said Leif-Eric Easley, associate professor of international studies at Ewha Womans University in Seoul. "Pyongyang could launch a provocation even when in desperate economic need because it wants to hide its weaknesses and extract external concessions."

Kim may also be going back to a tried-and-true technique of pressuring the world with missile launches and outrageous threats before offering negotiations at the last minute meant to extract aid.

"It bears further watching on how things go, but it's possible that we are near another phase in (North Korean) brinkmanship," said Park Won-gon, a professor of North Korea studies at Ewha Womans University.

North Korea's recent tests could be low-key reactions to the continuation of joint U.S.-South Korean military drills and to South Korean efforts to build up its arms programs.

Kim's ultimate goal is likely winning relief from crippling international economic sanctions even as he gains U.S. recognition as a nuclear state, allowing him to hold onto nuclear weapons that he may see as his only guarantee for survival.

WHAT'S NEXT?

North Korea may keep ramping up its pressure campaign, at least until China starts pushing for calm ahead of the Beijing Olympics early next year. But it still could hold back on tests of more provocative weapons as it looks for less coercive diplomacy.

The North will have until around November to advance its weapons development with testing, Park, the analyst, said. After that, it risks hurting relations with China.

North Korea may also conduct another weapons test around a major state anniversary, like the ruling Workers' Party foundation day on Oct. 10.

"For upgrading weapons capabilities, next in the testing queue may not be a nuclear device or ICBM but a submarine-based system," Easley said.

Despite its recent weapons tests, North Korea has maintained a self-imposed moratorium on nuclear and long-range missile tests for more than three years. That suggests that it still wants to keep chances for future diplomacy with Washington alive.

It's possible that Pyongyang is carefully measuring its actions while looking for a window back into diplomacy.

"It wouldn't be surprising if the North makes some effort soon to reach out to Washington or to Seoul, if just only to measure their intent," said Hong Min, an analyst at Seoul's Korea Institute for National Unification.

North Korean leader Kim Jong Un ordered his government to be prepared for both dialogue and confrontation with the Biden administration, but more for confrontation, state media reported Friday, days after the United States and others urged the North to abandon its nuclear program and return to talks.

Kim's statement indicates he'll likely push to strengthen his nuclear arsenal and increase pressure on Washington to give up what North Korea considers a hostile policy toward the North, though he'll also prepare for talks to resume, some experts say.

During an ongoing ruling party meeting Thursday, Kim analyzed in detail the policy tendencies of the U.S. under President Joe Biden and clarified steps to be taken in relations with Washington, the Korean Central News Agency said. It did not specify the steps.

Kim "stressed the need to get prepared for both dialogue and confrontation, especially to get fully prepared for confrontation in order to protect the dignity of our state" and ensure national security, it said.

In 2018-19, Kim held a series of summits with then-President Donald Trump to discuss North Korea's advancing nuclear arsenal. But the negotiations fell apart after Trump rejected Kim's calls for extensive sanctions relief in return for a partial surrender of his nuclear capability. 

Biden's administration has worked to formulate a new approach on North Korea's nuclear program that it describes as "calibrated and practical." Details of his North Korea policy haven't been publicized, but U.S. officials have suggested Biden will seek a middle ground between Trump's direct meetings with Kim and former President Barack Obama's "strategic patience" to curb Kim's nuclear program. 

Earlier this week, leaders of the Group of Seven wealthy nations issued a statement calling for the complete denuclearization of the Korean Peninsula and "the verifiable and irreversible abandonment" of North Korea's nuclear and missile programs. They called on North Korea to engage and resume dialogue. 

Sung Kim, the top U.S. official on North Korea, is to visit Seoul on Saturday for a trilateral meeting with South Korean and Japanese officials. His travel emphasizes the importance of three-way cooperation in working toward complete denuclearization of the Korean Peninsula, the State Department said.

Kim Jong Un has recently threatened to enlarge his nuclear arsenal and build high-tech weapons targeting the U.S. mainland if Washington refuses to abandon its hostile policy toward North Korea. 

In March, Kim's military performed its first short-range ballistic missile tests in a year. But North Korea is still maintaining a moratorium on long-range missile and nuclear tests in an indication that Kim still wants to keep prospects for diplomacy alive. 

Kwak Gil Sup, head of One Korea Center, a website specializing in North Korea affairs, wrote on Facebook that Kim's statement suggested he's taking a two-track approach of bolstering military capability and preparing for talks. But he said Kim will more likely focus on boosting military strength and repeating his demand for the U.S. to withdraw its hostile policy, rather than hastily returning to talks.

Kim said last week North Korea's military must stay on high alert to defend national security.

Analyst Cheong Seong-Chang at the private Sejong Institute in South Korea said North Korea will likely return to talks but won't accept a call for immediate, complete denuclearization. He said North Korea may accede to a proposal to freeze its atomic program and partially reduce its nuclear arsenal in phased steps if the Biden administration relaxes sanctions and suspends its regular military drills with South Korea.

Cha Duck Chul, a deputy spokesman at South Korea's Unification Ministry, said it's closely monitoring the North's ongoing political meeting and wants to reemphasize the best way to achieve peace on the Korean Peninsula is through dialogue.

In Beijing, Chinese Foreign Ministry spokesperson Zhao Lijiang called for renewed dialogue between North Korea and the U.S., saying that "We believe that the Korean Peninsula situation is facing a new round of tension."

Kim called the ruling Workers' Party's Central Committee meeting taking place this week to review efforts to rebuild the economy, which has been severely crippled by pandemic border closings, mismanagement amid the U.S.-led sanctions, and storm damage to crops and infrastructure last year.

On Tuesday, Kim opened the meeting by warning of potential food shortages, urging officials to find ways to boost agricultural production because the country's food situation "is now getting tense." He also urged the country to brace for extended COVID-19 restrictions, suggesting North Korea would extend its border closure and other steps despite the stress on its economy.