During his visit Wednesday to the nuclear sub plant, Kim bashed South Korea's recent push to develop its own nuclear-powered subs with the United States, calling it a "threat... that must be countered," the state-run Korean Central News Agency (KCNA) reported.

US President Donald Trump gave the green light for South Korea to build nuclear-powered subs during his trip to the country in October, sparking outrage from the nuclear-armed North.

However key details, such as where the vessels would actually be built, remain uncertain.

Only a handful of countries have nuclear-powered submarines, while the United States considers its technology among the most sensitive and tightly guarded military secrets.

Last month, in the North's first comments on the US-South Korea deal, a commentary piece by KCNA called the programme a "dangerous attempt at confrontation," and said it could lead to a "nuclear domino phenomenon."

Kim on Wednesday said that the current "negative security situation" made it an "urgent task and indispensable option" to more rapidly advance the "nuclear weaponization of the naval force," KCNA reported.

The North Korean leader also learned about ongoing research on "new underwater secret weapons" and "clarified a strategic plan for reorganizing the naval forces and establishing new units," KCNA said, without further details.

In a separate report, KCNA said Kim oversaw the test on Wednesday of "new-type high-altitude long-range anti-air missiles" over the East Sea, also known as the Sea of Japan.

"The launched missiles accurately hit the mock targets at an altitude of 200 km," KCNA said. That height, if correct, would be in space.

"The respected Comrade Kim Jong Un extended congratulations on the successful test-fire," KCNA said.