The top leader of the Democratic People's Republic of Korea (DPRK), Kim Jong Un, has announced that his country's next steps in developing its nuclear arsenal will be revealed at the upcoming ruling party congress, the official Korean Central News Agency (KCNA) reported on Wednesday.

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.

The DPRK 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 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 away, according to Kim.

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.

The test 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".

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". (AFP)