North Korea said Friday that it had tested an “underwater nuclear weapon system” in response to joint naval exercises by Washington, Seoul and Tokyo that involved a United States nuclear-powered aircraft carrier.

The drills were “seriously threatening the security” of the North, so in response, Pyongyang “conducted an important test of its 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 — a different version of the Haeil, which means tsunami in Korean — claiming it could unleash a “radioactive tsunami.”

Earlier this week, South Korea, the US 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 Friday the drills “constituted a cause of further destabilizing the regional situation, and they are an act of seriously threatening the security” of the North, the defense ministry spokesperson said, according to KCNA.