North Korea claims it 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 US 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 defence ministry carried by state news agency KCNA.

The North has claimed tests of its "Haeil-5-23" system before but the weapons have never been independently verified.

South Korea and allies have yet to respond to the latest provocation.