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.
Japan declined to comment on the reports on Friday.
The Nuclear Weapon Test
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.
Prior to the Test
That followed live-fire drills at the maritime border with South Korea in the first week of January.
Pyongyang leader Kim Jong Un has also been increasingly aggressive in his policy direction and rhetoric – ending several agreements aimed at peace-keeping in recent months.
On Friday, North Korea said it had been provoked by joint drills by Washington, Seoul and Tokyo to carry out a test of its underwater weapons, according to a report by state agency KCNA.
It accused the exercises of ‘further destabilising the regional situation’ and threatening the North’s security.
What US, South Korea and Japan Says About Their Drill
The US, South Korea and Japan say they have conducted more exercises in the past year as a deterrence response to the increasing frequency of North Korea’s military actions, which include multiple tests of its nuclear ballistic missiles and launches of new weapons.
All such actions are in breach of UN sanctions.
But Mr Kim has repeatedly said his regime is building up its military arsenal in preparation for war that could ‘break out at any time’.on the peninsula.
Over the New Year period, he signalled some fundamental policy shifts in his regime’s stance to South Korea.
And earlier this week he declared the former bedrock goal of re-unification with South Korea was over, designating the South as the ‘principal enemy’.
The rhetoric follows several claimed advances in his country’s military and nuclear capabilities – including in its underwater operations.