(SEOUL) North Korea said on Saturday it had conducted a new test of an underwater nuclear attack drone, in response to joint military exercises by South Korea and the United States.

The test of this “underwater strategic weapon system” was held from April 4 to 7, the official KCNA news agency said. The Haeil-2 type craft traveled a “simulated underwater distance of 1000 km”, she added.

North Korea announced on March 23 a first test of this unprecedented device, which it said could “produce a large-scale radioactive tsunami”.

The weapon “accurately exploded underwater,” KCNA added, saying the test “proves the reliability of the underwater strategic weapon system and its lethal attack capability.”

Seoul and Washington conducted joint aerial maneuvers on April 5, involving at least one American B-52H bomber capable of carrying nuclear weapons, according to the South Korean military.

Pyongyang considers that these exercises are rehearsals for an invasion of its territory and multiplies the tests of armaments. In particular, the regime claimed to have launched an intercontinental ballistic missile (ICBM) on March 16.

North Korea declared itself an “irreversible” nuclear power last year, and in March Kim Jong-un ordered his troops to step up their exercises for “real war.” He also called for accelerating the production of fissile material for nuclear weapons.

Satellite images taken a month ago showed a high level of activity at North Korea’s main nuclear complex, Yongbyon, according to the US-based organization 38 North.