The North tested the Haeil-2 underwater strategic weapon system from April 4-7, according to the Korean Central News Agency (KCNA) as quoted by Yonhap news agency.

"The system will serve as an advantageous and prospective military potential of the armed forces of the DPRK essential for containing all evolving military actions of enemies, removing threats and defending the country," the KCNA said in an English-language dispatch.

The North's provocation came as the secretive regime has vowed to take "overwhelming" actions against joint military drills between South Korea and the United States.

The test drone was deployed from a port in South Hamgyong Province on Tuesday, and "correctly set off" a test warhead underwater Friday after cruising along an "oval and eight-shaped" course simulating a distance of 1,000 kilometers for 71 hours and six minutes, it added.