North Korea said on Friday that its first submarine capable of launching nuclear missiles was now operational, a development that would give the country a new, harder-to-detect means of launching a nuclear strike.

The new “tactical nuclear attack submarine”, a remodelled Soviet-era vessel equipped with multiple launching tubes, was unveiled in a ceremony Wednesday, state media reported. In a speech, Kim Jong-un, the North’s leader, vowed to similarly convert more of its existing submarine fleet, calling it “an urgent task of the times” to “arm the navy with nuclear weapons”.

But South Korea’s military expressed scepticism about the submarine on Friday, saying that it “doesn’t look capable of normal operation” and that there were signs of “deception and exaggeration”.

The US and its allies have been closely watching the North’s attempts to develop a sub that could launch nuclear and other ballistic missiles. The capabilities of the one introduced this week, originally a Romeo-class Soviet submarine, are unknown; there is no evidence that North Korea has test-launched a missile from the vessel.

Photos released on Friday with the state media report show that the submarine has 10 vertical missile launch tubes of two different sizes, said Yang Uk, a weapons expert at the Asan Institute for Policy Studies in Seoul. It has an “abnormally large” missile launch deck for its size as if the North wanted to show off its nuclear force, Yang said.

That structure “will limit the submarine’s underwater stealthiness and manoeuvrability”, Yang added. “Still, the design reflects Kim Jong Un’s policy of increasing his nuclear force ‘exponentially​.’”

The submarine is powered by a diesel engine. ​That means that, unlike a nuclear-powered submarine, it would have to resurface frequently during a long-distance trip, like crossing the Pacific. For a distant adversary like the US, that makes it more detectable, and less of a threat, than a nuclear-powered sub.

Nevertheless, it could potentially pose a new threat to the North’s regional adversaries. Choi Il, a retired South Korean navy submarine captain, said it was designed to carry shorter-range ballistic and cruise missiles.