North Korea claimed on Friday to have tested an underwater nuclear weapon system under development in response to the recent joint naval exercise of South Korea, the United States and Japan.

In a statement published by the state-run Korean Central News Agency, the North’s defense ministry said the trilateral drill was a serious threat to national security.

The statement said that an important test of the mid-development “Haeil-5-23” underwater weapon in the East Sea was therefore conducted, but did not elaborate on the details of the test nor the outcome.

It went on to say that the military's underwater nuke-based countering posture is being further equipped, adding that maritime and underwater responsive actions will continue to deter the hostile military maneuvers by the navies of the U.S. and its allies.

From Monday to Wednesday, the three countries' navies conducted a joint maritime exercise aimed at enhancing their combined deterrence, involving the U.S. nuclear-powered aircraft carrier USS Carl Vinson, in international waters south from Jeju Island.