Pyongyang, North Korea - North Korea tested a new underwater nuclear weapons system earlier in the week, according to state news agency KCNA on Friday.

The agency said that North Korean leader Kim Jong-un oversaw drills from Tuesday to Thursday meant to demonstrate Pyongyang's "military attack capability" to Seoul and Washington and "verify the reliability of the nuclear force for self-defense."

Among the systems tested was the so-called "Unmanned Underwater Nuclear Attack Craft 'Haeil,'" which KCNA said is an "underwater strategic nuclear weapon system."

According to state media, the aim of the weapon is to sneak into "operational waters" and set off an underwater explosion to trigger a "radioactive tsunami" and destroy enemy naval forces and ports.

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KCNA said that the "underwater nuclear attack drone" was deployed on Tuesday from Riwon County on North Korea's east coast and set off an underwater detonation some 60 hours later in the waters off Hongwon Bay in the East Sea, as the Sea of Japan is also known.

North Korea also confirmed deploying several cruise missiles "tipped with a test warhead simulating a nuclear warhead" from its east coast on Wednesday.

Pyongyang said the tests, which have come at a furious pace over the past weeks, are a response to ongoing US military exercises with Seoul, which concluded on Thursday. Both countries deny Pyongyang's accusations that they are preparing for an attack.

The two allied countries, in turn, accuse North Korea of serious provocation.

UN resolutions prohibit North Korea from testing ballistic missiles of any range, which – depending on their design – can also be equipped with a nuclear warhead.

Observers fear that Pyongyang's first nuclear test in years is imminent.