The DPRK confirmed on Friday that it had tested an "underwater nuclear weapon system" in response to provocative joint naval exercises by Washington, Seoul, and Tokyo that involved a US nuclear-powered aircraft carrier.

The drills were "seriously threatening the security" of the DPRK, 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 defense ministry carried by state news agency KCNA.

Early last year, Pyongyang said it had carried out multiple tests of a purported underwater nuclear attack drone, emphasizing that it could unleash a "radioactive tsunami".

Earlier this week, South Korea, the United States, and Japan carried out joint naval drills in waters off southern Jeju Island, which they claimed were in response to the DPRK'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 Friday the drills "constituted a cause of further destabilising the regional situation, and they are an act of seriously threatening the security" of the country, the defense ministry spokesperson said, according to KCNA.

The DPRK'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 spokesperson said.

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

DPRK leader Kim Jong Un last week declared South Korea his country's "principal enemy", abolished government entities dedicated to reunification and outreach, and threatened war over "even 0.001 mm" of territorial infringement.

At Pyongyang's year-end policy meetings, Kim threatened a nuclear attack and called for a build-up of his country's military arsenal ahead of armed conflict he warned could "break out any time."

On Sunday, the DPRK launched a solid-fuel hypersonic missile, just days after it staged live-fire exercises near the country's tense maritime border with South Korea, which prompted counter-exercises and evacuation orders for some border islands belonging to the South.