North Korea tested a nuclear-capable unmanned underwater attack weapon called “Haeil-2” from April 4 to April 7, state media reported, more than a week after it disclosed a new underwater drone called “Haeil-1”, which translates as tsunami.

The North’s KCNA state news agency said that during the underwater strategic weapon system test the drone cruised 1,000 km (621 miles)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.

Some analysts are sceptical as to whether the underwater vehicle is ready for deployment but KCNA said the system would help contain military action against North Korea.

Photographs released by state media showed a large, dark-coloured torpedo-shaped object as well as tracks of the object’s underwater trajectory and blast visible on the surface of the sea.

North Korea has been regularly testing various weapons for months and ramped up its activity in recent weeks in response to joint U.S.-South Korean military exercises.

In recent weeks, it has unveiled new, smaller nuclear warheads and fired an intercontinental ballistic missile capable of striking anywhere in the United States.