In recent weeks, North Korea has tested what state media have described as an underwater nuclear-capable drone, and also carried out the launch of an intercontinental ballistic missile.

"A national defence science research institute in the DPRK carried out a test of underwater strategic weapon system from April 4 to 7," the official Korean Central News Agency said.

"The underwater nuclear attack drone 'Haeil-2'... cruised 1,000 km of simulated underwater distance."

KCNA added that "the test warhead accurately detonated underwater. The test perfectly proved the reliability of the underwater strategic weapon system and its fatal attack ability."

On March 23, North Korea had claimed it tested an underwater nuclear attack drone able to unleash a "radioactive tsunami", as it blamed US-South Korea exercises for a deteriorating regional security situation.

Satellite imagery has also indicated a high level of activity at North Korea's main nuclear complex after leader Kim Jong Un ordered the production of weapons-grade nuclear material be ramped up.

Last year, North Korea declared itself an "irreversible" nuclear power, while Kim has told the North Korean military to intensify drills in preparation for a "real war".

South Korea and the United States on Wednesday staged joint air drills involving at least one US nuclear-capable B-52H strategic bomber, Seoul's military said.

North Korea views such exercises as rehearsals for invasion, and has responded to other recent drills with a spate of increasingly provocative banned weapons tests.