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 defense 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 ... for 71 hours and 6 minutes.”

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.”

North Korea has claimed to have conducted three tests of underwater drones in less than three weeks so far.

On March 23, it claimed to have conducted the first test of the Haeil, which means tsunami in Korean, able to unleash a “radioactive tsunami” as it blamed
US-South Korea exercises for a deteriorating regional security situation.

Five days later it said it had carried out a second test.

In response South Korean Defense Minister Lee Jong-sup told MPs Seoul was “capable of monitoring and detecting such drones infiltrating underwater.”

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.

North Korea last year declared itself an “irreversible” nuclear power and Kim recently called for an “exponential” increase in weapons production, including tactical nuclear weapons.

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.

North Korea is seeking to diversify its delivery mechanisms in addition to increasing its nuclear stockpile.

Russia has also reportedly developed a similar weapon — nuclear-capable Poseidon torpedoes — but mastering the complex technology required for such weaponry might yet be beyond North Korea, experts said.

But the North’s claims about the tests should not be “easily dismissed for being exaggerated,” Choi Gi-il, professor of military studies at Sangji University, told AFP.

“While the North could have exaggerated the degree of success to some extent, they appear to show Pyongyang’s underlying confidence in this technology, some of which could have been transferred from Russia.”

Russia and North Korea have not officially commented on the transfer of the underwater drone technology, Choi added.