In recent weeks, Kim has declared South Korea his country's "principal enemy", jettisoned agencies dedicated to reunification and outreach, and threatened war over "even 0.001 mm" of territorial infringement.

He also recently hailed the test launch of a new strategic cruise missile from a submarine, calling it a key moment in the development of the North's naval power.

"The strengthening of the naval force presents itself as the most important issue in reliably defending the maritime sovereignty of the country and stepping up the war preparations at present," Kim said at the Nampho Dockyard, according to the Korean Central News Agency.

A nuclear-powered submarine was on Kim's strategic weapons wish list set out at a key party congress in 2021, along with a hypersonic warhead, spy satellites and solid-fuel intercontinental ballistic missiles.

During his visit to the Nampho shipbuilding base, about 65 kilometres southwest of Pyongyang, Kim was briefed on various warships under construction as well as preparations for a "new huge plan" assigned by the ruling party.

No details of the plan were provided.

Kim "expressed expectation that the workers of the dockyard would successfully build on the world level the major warships," KCNA said.

North Korea last year launched what it called its first "tactical nuclear attack submarine", which Seoul's military at the time said did not look to be operational.

Analysts said the vessel appeared to be modified from an existing diesel-electric submarine originally designed in the 1950s, and have posed questions about its limitations and vulnerabilities as a platform.