North Korea says tested 'underwater nuclear weapon system'

North Korea has tested an "underwater nuclear weapon system" in response to joint naval exercises by Washington, Seoul and Tokyo, which involved a US nuclear-powered aircraft carrier, state media said.

The drills were "seriously threatening the security" of the North, so in response, Pyongyang "conducted an important test of its underwater nuclear weapon system 'Haeil-5-23' under development in the East Sea of Korea," according to a statement from the Defence Ministry carried by state news agency KCNA.

North Korea has been stepping up pressure on Seoul in recent weeks, declaring it the "principal enemy," saying the North will never reunite with the South and vowing to enhance its ability to deliver a nuclear strike on the US and America's allies in the Pacific.

North and South Korea remain technically at war after the 1950-53 Korean War ended in a truce, not a peace treaty and tensions are running high.

Early last year, Pyongyang said it had carried out multiple tests of a purported underwater nuclear attack drone — a different version of the Haeil, which means tsunami in Korean — claiming it could unleash a "radioactive tsunami".

Analysts have questioned whether Pyongyang has such a weapon.

Trilateral drills

Earlier this week, South Korea, the United States and Japan carried out joint naval drills in waters off southern Jeju Island, which they said were in response to North Korea's Sunday launch of a hypersonic missile.

The drills involved nine warships from the three countries, including the aircraft carrier USS Carl Vinson.

Pyongyang said on Friday the drills "constituted a cause of further destabilising the regional situation, and they are an act of seriously threatening the security" of the North, the defence ministry spokesman said, according to KCNA.

North Korea's own test — the exact date of which was not given — ensured "our army's underwater nuke-based countering posture is being further rounded off and its various maritime and underwater responsive actions will continue to deter the hostile military maneuvers of the navies of the US and its allies," the spokesman said.

Recent months have seen a sharp deterioration in long-tense ties between the two Koreas, with both sides jettisoning key tension-reducing agreements, ramping up frontier security, and conducting live-fire drills along the border.

North Korean leader Kim Jong-un last week declared the South his country's "principal enemy", jettisoned agencies dedicated to reunification and outreach and threatened war over "even 0.001 mm" of territorial infringement.