Pyongyang carried out military drills of its own in response this week, the official Korean Central News Agency said, including test-firing a new nuclear-capable underwater drone.

The weapon's mission is to "stealthily infiltrate into operational waters and make a super-scale radioactive tsunami ... to destroy naval striker groups and major operational ports of the enemy," it said.

The new weapon, called Haeil which means tsunami in Korean, "can be deployed at any coast and port or towed by a surface ship for operation," the report said.

North Korean leader Kim Jong Un personally oversaw the tests, KCNA reported, and images released by Pyongyang's Rodong Sinmun newspaper showed a smiling Kim and what appeared to be an underwater explosion.

The agency also said Pyongyang had fired strategic cruise missiles "tipped with a test warhead simulating a nuclear warhead" on Wednesday.

But analysts questioned North Korea's claims.

The idea that Pyongyang has "a nuclear-capable underwater drone should be met with scepticism," said Leif-Eric Easley, a professor at Ewha University in Seoul.

"Pyongyang's claims about a new weapons system are not the same as a credible demonstration of capability," he added.

In a Twitter post, US-based analyst Ankit Panda said it could not be ruled out that the announcement was "an attempt at deception/psyop".

Even so, the claim was "shocking," Cheong Seong-chang of the private Sejong Institute told AFP.

If true, it is hard to see how Seoul "could respond to such a formidable new weapon from North Korea that (it says) can completely destroy the South's major operational ports."

The KCNA statement also indicates "Pyongyang is more than ready to use its tactical nuclear weapons at any time," An Chan-il, a defector-turned-researcher, told AFP.

"This obviously further strengthens Kim's justification for his future nuclear tests."

- Nuclear power? -

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.

"For an unmanned submarine to go deep underwater undetected, it requires advanced technology such as control sensors and radar," Choi Gi-il, professor of military studies at Sangji University told AFP.

North Korea has not acquired this "to an extent that it can deploy nuclear unmanned sea drones in combat yet," but its claims about the Haeil drone fit a broader pattern, Choi said.

Pyongyang has moved on from simply stockpiling nuclear warheads and is "attempting to further advance and diversify launch mediums," he said, adding that further testing could yet give Pyongyang "formidable" new ways to deliver a nuclear payload.

After a record-breaking year of weapons tests and growing nuclear threats from Pyongyang in 2022, Seoul and Washington have ramped up security cooperation.

On Thursday, the two allies completed their largest joint military drills in five years.

Pyongyang views all such exercises as rehearsals for invasion and on Friday claimed the recent exercises, dubbed Freedom Shield, were a drill for "occupying" North Korea.

Pyongyang's "underwater nuclear attack drone" drill was staged "to alert the enemy to an actual nuclear crisis," KCNA said.

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.

KCNA's Friday statement comes about a week after Pyongyang test-fired its largest and most powerful missile, a Hwasong-17 -- its second ICBM launch this year.