SEOUL - North Korea has tested a new nuclear-capable underwater attack drone, according to state media, as leader Kim Jong Un warned that joint military exercises between South Korea and the United States should be suspended.

The new North Korean drone cruised underwater for over 59 hours at a depth of 80 to 150 meters (260–500 feet) and detonated a non-nuclear payload in waters off its east coast on Thursday, according to North Korean state news agency KCNA.

Analysts say North Korea is showing off its increasingly diverse nuclear threats to Washington and Seoul, though they are sceptical about whether the underwater vehicle is ready for deployment.

Ankit Panda, the senior fellow at the American Carnegie Endowment for International Peace, stated that North Korea wants to send a message to the US and South Korea that there are many potential nuclear weapons delivery vectors that the allies would need to be concerned about and target in the event of a war.

"Silos, railcars, submarines, and mobile missile launchers on wheels would all be present. And now they're mixing in this submerged torpedo, " he said.

On Monday, the isolated country flew a short-range missile from a buried silo, a departure from usual basing methods.

According to the KCNA, the new drone system, known as "Haeil" or "Tsunami, is designed to launch covert operations in hostile waters and destroy naval strike groups and important operational ports by generating a sizable radioactive wave through an underwater explosion.

"This nuclear underwater attack drone can be deployed at any coast and port or towed by a surface ship for operation," the news agency said, adding that Kim oversaw the test.

A military official from South Korea stated that they were examining North Korea's assertions. There were no signs of a nuclear test, according to a U.S. official who spoke on the record.

North Korea may not have fully developed the miniature nuclear warheads required to fit on its more compact weapons. Analysts say perfecting such warheads would most likely be a key goal if the North resumed nuclear testing.

State media published a picture of Kim grinning next to a sizable torpedo-shaped object, but it did not specify that it was the new drone. Other images displayed the object's underwater trajectory as tracks and surface-level explosions.

Panda claimed that the weapon's design was akin to the Poseidon nuclear torpedoes, a brand-new class of retaliatory weapon intended to produce destructive, radioactive explosions in coastal areas, which are used by Russia.

On Friday South Korean President Yoon Suk Yeol said he would make sure North Korea paid for its "reckless provocations", during a speech to commemorate service members who died in clashes with North Korea in western waters, including a 2010 sinking of a navy ship that South Korea said was struck by a North Korean torpedo.

North Korea also said it fired cruise missiles on Wednesday to practice tactical nuclear strikes, confirming earlier reports from the South Korean military.

According to KCNA, the cruise missile was fitted with an "experimental warhead simulating a nuclear warhead" and had a range of 1,500 to 1,800 kilometers (930 to 1,120 miles).

The latest tests come as South Korean and US militaries kick off their biggest amphibious exercises in years on Monday, involving a US amphibious assault ship.

North Korea said military exercises by the United States and South Korea force its forces to "prepare for all-out war and build up nuclear forces both qualitatively and quantitatively on the basis of priority." . 

South Korean and American military drills have long incensed Pyongyang, which claims they are a prelude to an invasion of the North.

The United States and South Korea have criticized the North's tests as unstable and in violation of U.N. sanctions, while claiming that the exercises are purely defensive.

On Thursday, the allies wrapped up Freedom Shield 23, an 11-day springtime exercise, but other field training drills are still going on.

According to KCNA, North Korean leader Kim declared "his will to make the U.S. imperialists and the South Korean puppet regime plunge into despair for their choice," and warned the adversaries to halt careless anti-North Korean war drills.

Although North Korean leader Kim does not appear prepared to conduct a nuclear test during military drills between the United States and South Korea, the United States is maintaining vigilance, according to the director of the U.S. Defense Intelligence Agency on Wednesday.