North Korea 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.
The launch came hours after the US reaffirmed its offer to resume diplomacy on North Korea's nuclear weapons programme.
It underscored how the North continues to expand its military capabilities amid a pause in diplomacy.
The South's Joint Chiefs of Staff said in a statement it detected the North firing one short-range missile it believed was a submarine-launched ballistic missile (SLBM) from waters near the eastern port of Sinpo, and that the South Korean and US militaries were closely analysing the launch.
The South Korean military said the launch was made at sea, but it did not elaborate whether it was fired from a vessel submerged underwater or another launch platform above the sea's surface.
Japan's military said its initial analysis suggested the North fired two ballistic missiles and Prime Minister Fumio Kishida said officials were examining whether they were SLBMs.
After the launch, Mr Kishida interrupted a campaign trip ahead of Japanese legislative elections later this month, returning to Tokyo.
The leader ordered his government to start revising the country's national security strategy to adapt to North Korea's growing threats.
"We cannot overlook North Korea's recent development in missile technology and its impact on the security of Japan and in the region," he said.
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 accused Seoul of hypocrisy for criticising the North's weapons tests while expanding its own conventional military capabilities.
The apparent site of the missile firing - a shipyard in Sinpo - is a major defence 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 multiple missiles.
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.
Chinese Foreign Ministry spokesman Wang Wenbin said the tensions on the Korean Peninsula were at a "critical stage" and called for a renewed commitment to diplomatically resolving the issue.
Ending a months-long 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 he wants a more survivable nuclear deterrent able to blackmail his neighbours and the United States," said Leif-Eric Easley, a professor of international studies at Ewha Womans University in Seoul.
He added that North Korea "cannot politically afford appearing to fall behind in a regional arms race" with its southern neighbour.
"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 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 major 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, US president Joe Biden's special envoy for North Korea, Sung Kim, is scheduled to meet with US 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 US-led sanctions against North Korea and the North's denuclearisation 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 negotiation room as the Biden administration is intent on keeping sanctions in place until the North makes concrete steps toward denuclearisation.
"The US continues to reach out to Pyongyang to restart dialogue. Our intent remains the same. We harbour no hostile intent toward the DPRK and we are open to meeting without preconditions," Sung Kim told reporters on Monday.
The US Indo-Pacific Command said North Korea's latest launch did not pose an immediate threat to US personnel, territory, or that of its allies.
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