SEOUL — North Korea said Friday its new submarine has nuclear attack capabilities after years of development. Leader Kim Jong-un described the milestone as crucial in his efforts to build a nuclear-armed navy to counter the United States and its Asian allies.

The North’s official Korean Central News Agency said the vessel, named Hero Kim Kun Ok, is designed to launch tactical nuclear weapons from underwater but did not specify the number of missiles it could carry and fire.

South Korean officials expressed skepticism that the submarine would work as North Korea described and said it likely wasn’t ready for operational duty. Still, the vessel’s development underscored how the North is trying to extend the range of its nuclear arsenal with systems that are harder to detect in advance.

Based on Mr. Kim’s comments and photos by North Korean state press, it’s likely the submarine is the same one the communist ruler inspected in 2019 while it was under construction. At the time, experts assessed it as an effort to convert an existing Romeo-class submarine.

The submarine appears to have at least 10 launch tubes — four of them apparently larger than the other six — that are possibly designed for missiles.

“This submarine, though heavily modified, is based on 1950s Soviet-origin technology and will have inherent limitations. Nevertheless, in terms of complicating the targeting challenges that the U.S. and its allies will face, the submarine will serve North Korea’s purposes,” an analyst with the Carnegie Endowment for International Peace, Ankit Panda, said.

In recent years, North Korea has tested a variety of missiles designed to be fired from submarines as it pursued the ability to conduct nuclear strikes from underwater. In theory, such capacity would bolster its deterrent by ensuring a survivable capability to retaliate after absorbing a nuclear attack on land.

Ballistic missile submarines would also add a maritime threat to the North’s growing collection of solid-fuel weapons fired from land vehicles that are designed to overwhelm missile defenses in South Korea and Japan.

Even so, it would take considerable time, resources and technological improvements for the heavily sanctioned nation to build a fleet of at least several submarines that could travel quietly and execute attacks reliably, analysts say.

Satellite photos analyzed by the Associated Press on Friday placed the submarine and the ceremony that Mr. Kim took part in the eastern port city of Sinpo, where North Korea runs a major shipyard developing submarines.

The images, taken just before 8 a.m. local time Thursday by Planet Labs PBC, showed the submarine alongside a dock. The submarine appeared to bear the red-white-and-blue bunting seen in photographs released by North Korean state press on Friday. Other details of the state-released photographs matched known features surrounding the facility.

The submarine still appeared to be at port Friday morning, according to other images taken by Planet. The James Martin Center for Nonproliferation Studies first identified the submarine’s location.

In speeches at the vessel’s launching ceremony Wednesday and an onboard inspection Thursday, Mr. Kim expressed satisfaction that the country has acquired its own nuclear attack submarine to counter America’s advanced naval asset, KCNA said. In July, the U.S. docked a nuclear-capable ballistic missile submarine in South Korea for the first time since the 1980s.

Mr. Kim said the country is also pursuing a nuclear-propelled submarine and plans to remodel its existing submarines and surface vessels so they can handle nuclear weapons, describing the building of a nuclear-capable army as an “urgent task.”

South Korea’s military said North Korea was exaggerating the submarine’s capabilities. The South’s Joint Chiefs of Staff said the North would have needed to increase the size of the bridge and other parts of the original vessel to accommodate missile launch systems, but that the appearance of the new submarine suggested that it could “not be operated normally.”

“There are signs of deception or exaggeration,” the Joint Chiefs of Staff said in a statement, without elaborating.

Mr. Kim in recent weeks has been emphasizing the need to strengthen the country’s navy. Some analysts say the focus could be driven by a desire to expand military cooperation with Russia, which has suggested the possibility of combined military exercises between the countries.

There’s also talk that Mr. Kim is preparing to visit Russia for a meeting with President Putin to discuss North Korean arms sales to refill Russian reserves drained by its war on Ukraine.

In exchange for providing Russia with artillery shells and other ammunition, North Korea could seek badly needed economic aid and also advanced weapons technologies, including those related to submarine-launched ballistic missile systems, intercontinental ballistic missiles and military spy satellites, analysts say.

Russia — which has always closely guarded its most important weapons technologies, even from key allies like China — could be unwilling to exchange major technology transfers with North Korea for what’s likely to be limited war supplies transported over a small rail link between the countries.

Nuclear-propelled submarines, which can quietly travel long distances and approach enemy shores to deliver strikes, are among a long list of advanced weapons systems Mr. Kim has been openly pursuing as he attempts to build a nuclear arsenal that could viably threaten the American mainland. But such vessels are going to be largely unfeasible for the North without external assistance in the near-term, Mr. Panda said.

“Russian assistance could be quite helpful here, but it’s unclear if Moscow would be willing to share this sensitive technology,” he said.

Mr. Kim insisted that the submarine revealed this week would be just as daunting to his enemies as the nuclear-propelled submarine North Korea plans to acquire in the future.

“The nuclear attack submarine, for decades a symbol of aggression against our republic, has now become a symbol of our revolutionary power to strike fear into the hearts of our despicable enemies,” KCNA quoted Mr. Kim as saying.

North Korea previously had only one known submarine capable of firing a missile, but that vessel has a single launch tube and analysts considered it a test platform.

North Korea has an estimated 70 to 90 diesel-powered submarines in one of the world’s largest submarine fleets. But they are mostly aging subs capable of launching only torpedoes and mines, not missiles.

KCNA said the launching of the submarine was timed for the 75th anniversary of the country’s founding, which falls on Saturday and was to be marked by festivities, including a paramilitary parade.