North Korean leader Kim Jong-un, third from right, smiles during his visit to a shipyard to inspect nuclear submarine construction in this photo released by the North's official Korean Central News Agency, Saturday. Yonhap

North Korea has publicly unveiled the construction of a nuclear-powered submarine, a weapons system that could pose a serious security threat to South Korea and allies if successfully developed, due to its ability to operate submerged for months.

On Saturday, Pyongyang's state-run media, the Korean Central News Agency (KCNA), released photos of leader Kim Jong-un inspecting shipyards focused on constructing warships, including "a nuclear-powered strategic guided missile submarine."

"Kim stressed the need to make overwhelmingly powerful warships to contain hostile forces' habitual gunboat diplomacy," the KCNA reported without disclosing the exact location of the shipyards.

It was the first time that North Korea disclosed the ship's appearance.

Military and North Korea analysts believe this is among Kim's top military priorities that he laid out at a Workers' Party congress in early 2021. They said the naval vessel appears to be a 5,000-ton-class or 6,000-ton-class — twice the size of the Hero Kim Kun Ok, known as the North's first ballistic missile submarine that is about 3,000-ton class.

"Given the size and shape of the submarine, it's believed to be a ballistic missile submarine (SSBN)," said Yang Uk, a defense expert and research fellow at Asan Institute for Policy Studies.

Kim Dong-yup, a professor at the University of North Korean Studies, said the North wants to showcase its new nuclear-powered submarine as 2025 marks the final year of its five-year development plan.

"North Korea revealed its nuclear-capable Pukguksong-5 submarine-launched ballistic missile (SLBM) during a military parade in April 2022," he said. "In parallel with the development of the SLBMs, the reclusive regime has also been working simultaneously on its nuclear-powered submarine platform."

North Korea unveiled a shipyard that builds warships at an undisclosed location in this photo provided by the state-run Korean Central News Agency, Saturday. Yonhap

However, experts' views were divided on whether North Korea may have received Russian technological assistance in building a small nuclear reactor for the submarine, in exchange for supplying conventional weapons to support Russia's war against Ukraine.

"The released photos suggest that the submarine has been under construction for more than two years. North Korea may have received technology from Russia in the latter half of 2023, while supplying artillery shells and missiles in return to aid Russia's war against Ukraine," Yang said.

Kim, on the other hand, expressed skepticism about Russia sharing sensitive technology with Pyongyang.

"Although Russia may provide some specialists to assist with inspections or modifications, they are likely to be very cautious when it comes to core military technologies," he said.

Kim further noted that Pyongyang has the technical and engineering capabilities to build a nuclear submarine, although they do not appear to be perfected.

"Since the 1960s and 1970s, North Korea has made great progress by importing Soviet and Russian technology, reverse-engineering it and combining it with homemade technology," he said.

The experts also pointed out that it would take at least several more years to operate the submarine submerged.

"It would take at least four to five more years to deploy the submarine into the Pacific Ocean as it still needs to conduct more procedures including testing," Yang said.

The Ministry of Unification said it is closely monitoring any signs of Russia's transfer of technologies to North Korea, without making any prejudgment.

"The transfer of military technology to North Korea is prohibited under U.N. Security Council resolutions. Russia, as a permanent member of the U.N. Security Council, must abide by U.N. sanctions on North Korea," unification ministry spokesperson Koo Byung-sam said in a press briefing.

On Monday, North Korea fired multiple ballistic missiles as South Korea and the U.S. launched their annual Freedom Shield exercise that will run until March 20. This followed the North's threat that Seoul and Washington will pay a "horrible price for their dangerous provocative act."