SEOUL--North Korea on Tuesday fired at least one ballistic missile into the sea in what South Korea’s military described as a weapon likely designed for submarine-based launches, marking one of the most significant demonstrations of the North’s military might since President Joe Biden took office.

The launch came hours after the U.S. reaffirmed its offer to resume diplomacy on North Korea’s nuclear weapons program. It underscored how the North continues to expand its military capabilities amid a pause in diplomacy.

The South’s Joint Chiefs of Staff said it detected the North firing one short-range ballistic missile it presumed as an SLBM from waters near the eastern port of Sinpo, and that the South Korean and U.S. militaries were closely analyzing the launch.

The South Korean military said the launch was made at sea, but it didn’t elaborate whether it was fired from a vessel submerged underwater or another launch platform above the sea’s surface.

But Japanese Defense Minister Nobuo Kishi said his country’s initial analysis suggested that the North fired two ballistic missiles. Japan’s coast guard issued a maritime safety advisory to ships but didn’t immediately know where the alleged missiles landed.

The shipyard in Sinpo is a major defense industry hub where North Korea focuses its submarine production. In recent years, the North has also used Sinpo to develop ballistic weapons systems designed to be fired from submarines.

North Korea had last tested an SLBM in October 2019.

Analysts had expected the North to resume tests of such weapons after it rolled out at least two new SLBMs during military parades in 2020 and 2021. There have also been signs that the North is trying to build a larger submarine that would be capable of carrying and firing more than one missile at a time.

South Korean officials held a national security council meeting and expressed “deep regret” over the launch that came despite efforts to revive diplomacy. A strong South Korean response could anger North Korea, which has been accusing Seoul of hypocrisy for criticizing the North’s weapons tests while expanding its own conventional military capabilities.

Japan’s Deputy Chief Cabinet Secretary Yoshihiko Isozaki said Tokyo has lodged a “strong protest” to North Korea through the “usual channels,” meaning their embassies in Beijing. Japan and North Korea have no diplomatic ties.

Ending a monthslong lull in September, North Korea has been ramping up its weapons tests while making conditional peace offers to Seoul, reviving a pattern of pressuring South Korea to try to get what it wants from the United States.

North Korean leader Kim Jong Un is "developing submarine-launched ballistic missiles because it wants a more survivable nuclear deterrent able to blackmail its neighbors and the United States,” said Leif-Eric Easley, a professor of international studies at Ewha Women’s University in Seoul.

Easley added that North Korea “cannot politically afford appearing to fall behind in a regional arms race” with its southern neighbor.

“North Korea’s SLBM is probably far from being operationally deployed with a nuclear warhead," he added.

North Korea has been pushing hard for years to acquire an ability to fire nuclear-armed missiles from submarines, the next key piece in Kim Jong Un’s nuclear arsenal that includes a broad range of road mobile missiles and ICBMs with potential range to reach the American homeland.

Still, experts say it would take years, resources and technological improvements for the heavily sanctioned nation to build a fleet of at least several submarines that could travel quietly in seas and reliably execute strikes.

Within days, Biden’s special envoy for North Korea, Sung Kim, is scheduled to meet with U.S. allies in Seoul over the prospects of reviving talks with North Korea.

Nuclear negotiations between Washington and Pyongyang have stalled for more than two years over disagreements in exchanging the release of crippling U.S.-led sanctions against North Korea and the North’s denuclearization steps.

But while North Korea is apparently trying to use South Korea’s desire for inter-Korean engagement to extract concessions from Washington, analysts say Seoul has little wiggle room as the Biden administration is intent on keeping sanctions in place until the North makes concrete steps toward denuclearization.

“The U.S. continues to reach out to Pyongyang to restart dialogue. Our intent remains the same. We harbor no hostile intent toward the DPRK and we are open to meeting without preconditions,” Sung Kim told reporters on Monday, referring to the North’s official name, the Democratic People’s Republic of Korea.

Last week, Kim Jong Un reviewed powerful missiles designed to launch nuclear strikes on the U.S. mainland during a military exhibition and vowed to build an “invincible” military to cope with what he called persistent U.S. hostility. Earlier, Kim dismissed U.S. offers for resuming talks without preconditions as a “cunning” attempt to conceal its hostile policy on the North.

The country has tested various weapons over the past month, including a new cruise missile that could potentially carry nuclear warheads, a rail-launched ballistic system, a developmental hypersonic missile and a new anti-aircraft missile. The test of the hypersonic missile on Sept. 28 came shortly before North Korean Ambassador Kim Song called for the Biden administration to permanently end joint military exercises with South Korea and the deployment of strategic military assets to the region in his speech to the U.N. General Assembly.