North Korea has ramped up its military activities in recent weeks amid US-South Korea military drills.
North Korea carried out an underwater strategic weapon system test from April 4 to April 7, state media KCNA said on Saturday.
The country tested another kind of a nuclear-capable unmanned underwater attack weapon called "Haeil-2", after it disclosed a new underwater drone system designed to make sneak attacks in enemy waters.
"The test perfectly proved the reliability of the underwater strategic weapon system and its fatal attack ability," the North's state news agency said.
North Korea has ramped up its military activities in recent weeks in a show of force against the United States and South Korea that conduct joint military drills.
It unveiled new, smaller nuclear warheads and fired an intercontinental ballistic missile capable of striking anywhere in the United States.
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