North Korea said on Saturday that it was building its first nuclear-powered submarine to enhance its nuclear weapons abilities, and the state news media showed the nation’s leader, Kim Jong-un, inspecting part of what appeared to be a new submarine larger than any owned by the country.

The announcement, made in the state news media, came as United States and South Korean officials feared that the North may be receiving technological help from Russia to help modernize its military in return for sending troops and conventional weapons to help in its war against Ukraine.

Mr. Kim “learned about the building of a nuclear-powered strategic guided missile submarine,” the North’s official Korean Central News Agency said in a report on Saturday about his visit to an unidentified naval shipyard. The agency carried photographs showing what appeared to be the hull of a submarine under construction.

It was the first time that North Korea claimed that a nuclear-powered submarine was under construction and claimed to show a photograph of it.

North Korea has dozens of old Soviet-era submarines, all diesel-powered, which would have to resurface frequently to recharge their batteries during a long-distance trip like crossing the Pacific. A nuclear-powered submarine, which can cover a long distance without resurfacing, would enhance North Korea’s ability to approach a faraway enemy like the United States without being detected, and to strike it with nuclear missiles.

The state news media on Saturday quoted Mr. Kim as saying that his country’s naval might would be “fully displayed in any necessary waters without limitation.”