North Korea has tested an undersea nuclear attack device capable of triggering a “massive radioactive tsunami”.

According to the North Korean news agency, Kim Jong-un oversaw the tests.

North Korea said on Friday it had tested an unmanned underwater nuclear attack device capable of triggering a “radioactive tsunami” and blamed US-South Korea military exercises for destabilizing regional security.

This week, Pyongyang also conducted military maneuvers, including testing a new underwater nuclear-powered missile launcher, North Korea’s state news agency KCNA reported.

“This submarine nuclear attack device can be placed on any coast or port or towed by a surface ship,” KCNA reported.

The purpose of the device, he added, was to “stealth penetrate operational waters and create a large-scale radioactive tsunami … to destroy enemy naval strike groups and key operational ports.”

North Korean leader Kim Jong-un personally oversaw the tests, KCNA reported. On Wednesday, Pyongyang also launched strategic cruise missiles “equipped with a test warhead simulating a nuclear warhead,” the agency said.

However, some analysts questioned North Korea’s claims.

“The idea that Pyongyang has a nuclear-capable submarine warship should be viewed with skepticism,” said Leif-Erik Eisley, a professor at Ewha University in Seoul.

In a post on the social network Twitter, American researcher Ankit Panda did not rule out the allegation of the experiment as “an attempt to deceive / psychological manipulation”.

However, Cheong Seong-chang of the Sejong Institute told Agence France-Presse that the allegation was “shocking”.

If true, it is difficult to see how Seoul could respond to such an incredible new weapon from North Korea that could completely destroy South Korea’s main operational ports,” he said.

Seoul says North Korea will pay for reckless provocations

South Korea’s president vowed that North Korea would “pay for its reckless provocations”, hours after Pyongyang announced it had conducted tests of a new type of underwater nuclear attack device.

Yoon Suk-yeol speaks at a memorial service for 55 sailors killed in the Yellow Sea in 2002 during the so-called Second Yongpyeong War, and following a 2010 torpedo fired by a North Korean submarine against the corvette Cheonan.

“North Korea is advancing day by day with nuclear weapons and carrying out provocations with unprecedented intensity,” Yoon said, speaking at a cemetery in Daejeon, 139 kilometers south of Seoul.

“Our government and military will dramatically strengthen South Korea’s three-pronged system in the face of missile advances and provocations from the North, and strengthen defense cooperation with the United States and Japan.”