North Korea says tested ‘underwater nuclear weapon system’

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.”