The launch was the latest in a string of sanctions-busting North Korean weapons tests so far this year, and came after US and South Korean officials warned Pyongyang was preparing to resume nuclear testing.

"One ballistic missile fired by North Korea today at 1203 (0303 GMT) from around Sunan towards the East Sea (Sea of Japan) was detected," South Korea's Joint Chiefs of Staff said in a statement.

"Currently, our military is maintaining readiness posture by tracking and monitoring related movements in preparation for additional launches."

Japan's Coast Guard also said that North Korea had launched "potentially a ballistic missile".

The nuclear-armed state staged a dramatic return to long-range launches in March, test-firing at full range its most powerful intercontinental ballistic missile -- which may be able to reach the continental United States.

Such tests had been paused while Kim met then-US President Donald Trump for a bout of diplomacy that collapsed in 2019.

Talks have stalled since, and despite biting sanctions, North Korea has doubled down on its military modernisation drive.

- More nukes? -

Kim Jong Un said at a military parade last week that he would take measures to develop "the nuclear forces of our state at the fastest possible speed", according to footage of his speech broadcast on state media.

"The nuclear forces, the symbol of our national strength and the core of our military power, should be strengthened in terms of both quality and scale."

Repeated negotiations aimed at convincing Kim to give up his nuclear weapons programmes have come to nothing.

"There is a good chance that they test-fired a missile that can be equipped with a nuclear warhead," Ahn Chan-il, a North Korean studies scholar, told AFP.

Kim also warned that he could "pre-emptively" use his nuclear force to counter hostile forces at a meeting with top military brass last week.

The latest weapons test came just days before South Korea's incoming president Yoon Suk-yeol, who has vowed a tougher stance on the North, takes office next week.

"It could be a warning message to... Yoon," said Hong Min of the Korea Institute for National Unification.

Yoon has hinted he is only willing to talk about peace if North Korea confirms it is willing to denuclearise -- something Pyongyang will never accept, Hong Min said.

"It could also signal Pyongyang's stance that it has no choice but to further enhance its arsenal if Seoul and Washington decided to deploy strategic military assets to the South," he added.

US President Joe Biden is due to visit South Korea in May.