North Korea says it tested new nuclear underwater attack drone

North Korean state news agency KCNA said on Friday it tested a new nuclear underwater attack drone under leader Kim Jong Un's guidance this week, as a U.S. amphibious assault ship arrived in South Korea for joint drills.

The North's state news agency also confirmed it fired cruise missiles during the weapon test and firing drill that took place from Tuesday to Thursday.

During the drill, the North Korean drone cruised underwater for over 59 hours and detonated in waters off its east coast on Thursday, the KCNA said. It did not elaborate on the drone's nuclear capabilities.

The drone system is intended to make sneak attacks in enemy waters and destroy naval striker groups and major operational ports, the KCNA said.

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"This nuclear underwater attack drone can be deployed at any coast and port or towed by a surface ship for operation," the news agency said.

In a separate firing drill, North Korea also confirmed it fired four cruise missiles on Wednesday to practice carrying out tactical nuclear attack missions.

The missiles were tipped with a "test warhead simulating a nuclear warhead," and flew 1,500 to 1,800 kilometres, it added.

The South Korean military has said North Korea fired four cruise missiles off its east coast on Wednesday.

The North said the latest weapon test and drills had no negative impact on security of the neighbouring countries.