North Korea says it has just wrapped up three days of tests on a nuclear-capable underwater attack drone.

Pyongyang made the move as the United States and South Korea performed their largest joint military exercises in years.

North Korean media say the drone cruised for nearly 60 hours at a depth of 80 to 150 meters before it was detonated in the Sea of Japan.

The Rodong Sinmun newspaper claims the weapon will be able to infiltrate waters and create a radioactive tsunami able to destroy naval striker groups and enemy ports.

North Korea also reportedly test-fired four strategic cruise missiles on Wednesday from the country's east toward the Sea of Japan.

It claims they flew for more than two and a half hours along an oval and pattern-8 course and hit targets at sea that were 1,500 to 1,800 kilometers away.

The US and South Korea have just wrapped up drills to prepare for possible emergencies on the Korean Peninsula.

But the two countries are scheduled to conduct large-scale landing training through April 3.