North Korean leader Kim Jong Un inspected a naval base under construction, state media reported Thursday, calling it a "core undertaking" for maritime security.

The visit follows his pledge last week to possess "the most advanced and powerful warships", as the nuclear-armed state speeds up its ambitious naval expansion.

North Korea has been upgrading its navy in recent years, commissioning 5,000-tonne destroyers and testing ship-launched missiles.

Kim has pledged to arm the fleet with nuclear weapons and build larger 10,000-tonne warships.

He "gave field guidance at the construction of major naval base on the east coast, which is being accelerated", official Korean Central News Agency reported in an English-language dispatch.

"The construction of the modern naval base is a crucial core undertaking directly related to the security of our state and the defence of its maritime sovereignty from A to Z," he was quoted as saying.

"It should be completed without fail at the qualitative level", he added.

KCNA photos showed Kim walking around the seaside construction site, accompanied by naval officials. Another image showed him speaking to officials gathered around him.

Details of the new naval base including its exact location and scale were not reported.

On Monday, the North test-fired a new high-speed torpedo system, which is expected to enter naval service in early 2027.

The nuclear-armed state last year unveiled a 5,000-tonne destroyer-class vessel that experts believe can carry both ship-to-surface and ship-to-air missiles.