The underwater drone, which supposedly can carry a nuclear weapon, was tested off the east coast, state media said.

There is no other evidence of the tests being conducted and Seoul had earlier said the North’s descriptions of the drones’ capability were exaggerated.

South Korea called the reported tests a “provocation”.

It “threatens peace on [the] Korean Peninsula and the world,” the South Korea’s defence ministry said, adding: “If North Korea directly provokes us, we will respond in an overwhelming manner following the principle of immediate, strong, and terminal action.”

The North has claimed tests of its “Haeil-5-23” system before, but the latest incident comes as the North has ramped up military action in recent weeks.

On Sunday, it claimed to have deployed a new solid-fuelled intermediate-range ballistic missile.

That followed live-fire drills at the maritime border with South Korea in the first week of January.