North Korea fired a submarine-launched ballistic missile (SLBM) off its east coast on Tuesday, South Korea's military said, pulling Japan's new prime minister off the campaign trail and overshadowing the opening of a major arms fair in Seoul.

The U.S. military's Indo-Pacific Command condemned the launch as destabilising, but judged it did not pose an immediate threat to the United States or its allies.

South Korean and U.S. officials were investigating if the missile was a smaller, previously unseen version displayed last week at an exhibit in Pyongyang, a South Korean military source told Reuters, citing ongoing analysis.

They were also examining whether the missile was fired from North Korea's experimental Gorae-class submarine, he added, speaking on condition of anonymity.

The launch, reported by officials in South Korea and Japan, came after U.S. and South Korean envoys met in Washington on Monday to discuss the nuclear standoff with the North.

The launch is the latest weapons test by North Korea, which has persisted with military development despite international sanctions over its nuclear weapons and missile programmes.

"Our military is closely monitoring the situation and maintaining readiness posture in close cooperation with the United States, to prepare for possible additional launches," South Korea's Joint Chiefs of Staff said in a statement.

They said the missile was launched at about 10:17 a.m. from the sea in the vicinity of Sinpo, where North Korea keeps submarines as well as equipment for test firing SLBMs.

It was not immediately clear if the missile was fired from a submarine or from a submersible test barge, as in most previous tests. A military source said it flew about 430 km to 450 km (267 to 280 miles) to a maximum altitude of 60 km (37 miles).

North Korea's last SLBM test was in October 2019, when a Pukguksong-3 missile was fired from an underwater platform, flying 450 km to a maximum altitude of 910 km.

Kim Dong-yup, a former South Korea Navy officer who is a professor at Seoul's Kyungnam University, said the latest test probably involved an SLBM recently unveiled by North Korea.

The North displayed new Pukguksong-4 and Pukguksong-5 SLBMs during military parades in October and January respectively, and a previously unseen, smaller missile was spotted at last week's defence fair in Pyongyang.

North Korea has also been working on what would be its first operational submarine capable of launching an SLBM, South Korean officials say.

The Washington-based Nuclear Threat Initiative has said SLBM capability could provide North Korea with additional options for nuclear launch, and a hedge against destruction of land-based nuclear systems.

INTERNATIONAL STANDOFF

South Korea's national security council expressed regret over the test and urged the North to resume talks.

Japanese Prime Minister Fumio Kishida said two ballistic missiles had been detected, adding that it was regrettable that North Korea held a string of missile tests in recent weeks.

There was no immediate explanation from South Korea or Japan over the conflicting number of missiles detected.

Kishida cancelled scheduled campaign appearances in northern Japan, and the deputy chief cabinet secretary said he planned to return to Tokyo to deal with the missile situation.

Spy chiefs from the United States, South Korea, and Japan who were meeting in Seoul on Tuesday, shared information on the missile test and agreed to strengthen intelligence sharing, South Korea's National Intelligence Service said in a statement.

The U.S. special representative for North Korea, Sung Kim, said that he would visit Seoul for talks this week.

"The U.S. continues to reach out to Pyongyang to restart dialogue," Kim said after meeting his South Korean counterpart in Washington on Monday.

"We harbour no hostile intent towards (North Korea), and we are open to meeting with them without preconditions."

The recent launches as well as the unusual military show in its capital suggest North Korea may be looking again to military and world affairs after nearly two years of focusing on the COVID-19 pandemic, said Chad O'Carroll of Korea Risk Group.

"Pyongyang tends to focus on one big strategic issue at a time, so the renewed testing could suggest military - later foreign policy - now priority," he said on Twitter.

MISSILE RACE

The missiles tested recently by North Korea appear aimed at matching or surpassing an arsenal South Korea has been expanding quietly, analysts have said.

Its successful test last month of an SLBM made the South the first country without nuclear weapons to develop such a system. North Korea test fired a missile from a train the same day.

This month the neighbours held duelling defence exhibitions to showcase their latest weaponry amid a spiralling arms race.

News of Tuesday's missile launch broke as representatives of hundreds of international companies and foreign militaries gathered in Seoul for the International Aerospace and Defence Exhibition (ADEX), which organisers called its largest defence expo ever.

South Korea is also preparing to test fire its first homegrown space launch vehicle on Thursday.

Analysts say the rocket has few potential applications as a weapon, but such tests are unlikely to be welcomed by North Korea.