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 whether the underwater vehicle is ready for deployment.

North Korea intends to signal “to the United States and South Korea that in a war, the potential vectors of nuclear weapons delivery that the allies would have to worry about and target would be vast,” said Ankit Panda, senior fellow at the U.S.-based Carnegie Endowment for International Peace.

“There would be silos, railcars, submarines and road mobile missile launchers. And now they’re adding this underwater torpedo to the mix,” he said.

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

Dubbed “Haeil”, or tsunami, the new drone system is intended to make sneak attacks in enemy waters and destroy naval strike groups and major operational ports by creating a large radioactive wave through an underwater explosion, the KCNA said.

“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 South Korean military official said they were analysing North Korea’s claims. A U.S. official, speaking on the condition of anonymity, said there was no indication of a nuclear test.

It is unclear whether North Korea has fully developed miniaturised nuclear warheads needed to fit on its smaller weapons.

Analysts say perfecting such warheads would most likely be a key goal if the North resumes nuclear testing.