The congress meeting "will clarify the next-stage plans for further bolstering up the country's nuclear war deterrent," Kim was quoted as saying as he oversaw a ballistic missile test on Tuesday.

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

KCNA said Kim, accompanied by senior North Korean officials, attended the test-firing of a "large-caliber" multiple rocket launcher that fired four missiles.

While acknowledging that development of the rocket launcher system had "not been plain sailing", Kim said Tuesday's test was "of great significance in improving the effectiveness of our strategic deterrent."

The rockets "hit a target" in waters 358.5 kilometers (222.7 miles) away, according to the North Korean leader.

The ballistic missiles were fired toward the Sea of Japan, with two missiles landing outside the country's Exclusive Economic Zone, Japanese state news agency Jiji Press had reported, citing defence ministry sources.

"The result and significance of this test will be a source of excruciating mental agony and serious threat to the forces that attempt to provoke a military confrontation with us," Kim said.

Tokyo condemned Pyongyang for Tuesday's missile test, saying it violated UN Security Council resolutions and was a threat to the peace and safety of the region as well as Japan, the Kyodo News Service reported.

- Second test in January -

The test was 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 came 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".

Ties between the United States and South Korea, longstanding treaty allies, were forged in the bloodshed of the Korean War (1950-53).

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.

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

During his first term, US President Donald Trump met with Kim three times, in an effort 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 2025, but these went unanswered by the North Korean leader.