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 U.S. nuclear-powered aircraft carrier.

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 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."