In his address that day, Yoon warned the North against using nuclear weapons. “That day will see the end of the North Korean regime.”

North Korea may be building a new submarine, the South Korean defense ministry said citing intelligence indications in a report to a member of parliament. In January, Kim reportedly ordered a nuclear submarine to be built.

The construction was at an early stage and it was not clear if the vessel was a nuclear-powered submarine, said the report.

North Korea is also working on a submarine drone that could be developed to carry nuclear weapons, possibly with the help of Russia, it said.

KCNA said Kim made his “military superpower” remarks on Monday, the same day the North has said its Supreme People’s Assembly would meet to discuss amending the country’s constitution. The news agency has made no mention of the assembly’s deliberations since Monday.

The session is being closely watched because of the likelihood it would approve a constitutional amendment to reflect Kim’s statement that unification is no longer possible and the South was a separate country and “a principal enemy.”

Such a move would formalize Kim’s break with decades-old goal espoused by both countries of national unification and attempts to improve ties, including a 2018 summit where their leaders declared there will be no more war and a new era of peace has opened.

In a separate report, KCNA said Kim sent a birthday message to Russian President Vladimir Putin, calling him his “closest Comrade” and saying “strategic and cooperative relations” between the two countries will be raised to a new level.

South Korea’s Defense Minister Kim Yong-hyun said “there was a high possibility” North Korea could deploy troops to help Russia in the war with Ukraine.

Kim also told a parliament hearing news reports of North Korean military officers having been killed in a Ukrainian strike in Russian-occupied territory were likely true.

Kim Jong Un and Putin in June adopted on a comprehensive strategic partnership that includes mutual defense pact.

The two countries have denied accusations by U.S. and South Korean officials the North was supplying arms to Russia.