Pyongyang has significantly increased missile testing in recent years, aimed according to analysts at improving precision strike capabilities, challenging Washington as well as Seoul, and testing weapons before exporting them to key ally Russia.

The Japanese coast guard, citing the defence ministry, said it had detected two ballistic missiles fired towards the Sea of Japan.

Japanese news agency Jiji Press reported the two missiles had landed outside of the country's Exclusive Economic Zone, citing defence ministry sources.

South Korea's Joint Chiefs of Staff also said it had detected several ballistic missiles fired by the North towards what Seoul calls the East Sea.

The test is Pyongyang's second of the month, following a salvo of missiles fired hours before South Korea's leader headed to China for a summit.

It comes a day after a high-level visit to Seoul by the Pentagon's number three official Elbridge Colby, who hailed South Korea as a "model ally".

Longstanding treaty allies, ties between the United States and South Korea were forged in the bloodshed of the Korean War.

Washington still stations 28,500 troops in South Korea as a deterrent against the nuclear-armed North.

Pyongyang routinely denounces Washington and Seoul's joint military drills as rehearsals for invasion.

North Korean leader Kim Jong Un last month bashed Seoul's push to develop its own nuclear-powered submarines with the United States, calling it a "threat" that "must be countered".

- 'Heighten tensions' -

Trump met North Korea's Kim three times during his first term, in efforts to reach a denuclearisation deal.

But since a summit in Hanoi fell through over differences about what Pyongyang would get in return for giving up its nuclear weapons, no progress has been made between the two countries.

Trump had expressed hopes for a meeting with Kim ahead of a regional summit in South Korea in October, but these went unanswered by the North Korean leader.

Pyongyang meanwhile has dispatched thousands of troops to fight for Russia, according to South Korean and Western intelligence agencies, as Moscow presses ahead with its nearly four-year invasion of Ukraine.

North Korea is also set to hold a landmark congress of its ruling party in the coming weeks, its first in five years.

Ahead of that conclave, Kim ordered the "expansion" and modernisation of the country's missile production.

Yang Moo-jin, chair professor at the University of North Korean Studies in Seoul, told AFP that "with the party congress approaching, the latest launch appears intended to heighten tensions to reinforce internal discipline and consolidate regime unity."

He added that the timing of the launch "may also be a response to Colby's visit" to the peninsula.