Kim visited shipyards focused on building warships, the Korean Central News Agency reported, without giving details of the exact date or location of the inspection.

Kim "learned about the building of a nuclear-powered strategic guided missile submarine," the report said, which was one of Kim's key military goals on a laundry list of high tech weaponry unveiled at a previous party congress.

The report is likely referring to a nuclear-powered submarine capable of launching ballistic missiles, Yonhap news agency reported, adding this was the first time the nuclear-armed North had revealed construction of a ballistic missile submarine.

Kim said the country's "sea defence capability... will be fully displayed in any necessary waters without limitation," KCNA said.

"The development of the naval force into an elite and nuclear-armed force constitutes an important content in the strategy for the development of the national defence," it added.

In 2023, North Korean state media reported on the launch of the country's first "tactical nuclear attack submarine" -- although the South Korean military said at the time that the vessel might not be operational.

According to the Nuclear Threat Initiative, a US-based think tank, North Korea is estimated to have between 64 and 86 submarines, one of the world's largest fleets.

However, experts doubt if all of them are operational given their age, according to NTI.

Ties between Pyongyang and Seoul are at one of their lowest points in years, with the South accusing Kim Jong Un of sending thousands of soldiers to Russia to help Moscow fight Ukraine, violating rafts of sanctions on both countries.

Last week, North Korea carried out a test-launch of strategic cruise missiles in the Yellow Sea, in a drill Pyongyang said was aimed at showing off its "counterattack" capabilities.

Joint South Korea-US "Freedom Shield" military exercises are set to begin later this month, and the USS Carl Vinson, the flagship of a carrier strike group, arrived in Busan for a scheduled port visit on Sunday, prompting an angry retort from Pyongyang.

Washington and Seoul describe such exercises as defensive in nature, but Pyongyang claims they are rehearsals for invasion and has responded with weapons tests of its own.

The two Koreas remain technically at war since the 1950-1953 Korean War ended in an armistice, not a peace treaty.