Pyongyang has been developing submarine-launched ballistic missile (SLBM) technology for years.

It showed off four such devices at a military parade overseen by leader Kim Jong Un in January, with state media KCNA calling them "the world's most powerful weapon".

Days earlier, Kim told a congress of his ruling Workers' Party that the North had completed plans for a nuclear submarine.

Any such vessel is likely to be years away from going into service, but analysts say it could be a strategic game-changer, enabling Pyongyang to launch a surprise strike underwater even if its land-based forces had been destroyed.

The South held the commissioning ceremony for its indigenously-developed, diesel-powered 3,000-ton submarine on the southern island of Geoje, the Navy said in a press release.

The vessel was named after revered independence activist Ahn Chang-ho and Rear Admiral Yang Yong-mo described it as a strategic asset that "will firmly defend our seas and its mere existence will strike fear" into the enemy.

The latest addition to the submarine fleet is 83.5 metres (274 feet) long and 9.6 metres (31 feet) wide and can stay underwater for 20 days without surfacing.

It reportedly comes with six vertical launching tubes to fire SLBMs.

Defence authorities plan to deploy it by August next year after a year-long evaluation.

The North is also looking to enhance its submarine forces.

Leader Kim inspected a newly built submarine in 2019, when pictures showed him standing next to a gigantic vessel accompanied by officials.

State media said it would soon be deployed for operations, without giving details of its capabilities.

Friday's commissioning ceremony came after Pyongyang slammed Seoul for going ahead with joint military exercise with the US.

Washington stations around 28,500 troops in the South to help defend it against its neighbour, which invaded in 1950, sparking the Korean War.

The US and South Korean militaries began their preliminary training Tuesday in the run-up to next week's yearly summertime exercise.

Pyongyang considers the drills a rehearsal for invasion and regularly condemns them.