The country tested another kind of a nuclear-capable unmanned underwater attack weapon called “Haeil-2”, over a week after it disclosed a new underwater drone system dubbed “Haeil-1”, which translates to tsunami in Korean, designed to make sneak attacks in enemy waters.

Analysts say North Korea is displaying its diverse nuclear delivery capabilities against Washington and Seoul, though they are sceptical whether the underwater vehicle is ready for deployment.

During the underwater strategic weapon system test from April 4 to April 7, state media KCNA said that the drone cruised 1,000km of underwater distance for 71 hours and 6 minutes and successfully hit a simulated target.

“The test perfectly proved the reliability of the underwater strategic weapon system and its fatal attack ability,” KCNA said.

North Korea has ramped up its military activities in recent weeks, protesting that the US and South Korea conduct joint military exercises.

It unveiled new, smaller nuclear warheads and fired an intercontinental ballistic missile capable of striking anywhere in the US.