North Korea has tested a new nuclear-capable underwater attack drone, state media reported on Friday, as leader Kim Jong Un warned joint military drills by South Korea and the U.S. should stop.

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

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.