North Korea appears to be developing a new weapons system capable of launching submarine-based ballistic missiles, the South's defence ministry said Monday.

"Based on recent US and South Korean intelligence, we have detected signs of North Korea developing a vertical missile launch tube for submarines," a ministry official told AFP.

Ministry spokesman Kim Min-Seok told a regular press briefing Monday that the North's 3,000-ton Golf-class submarine could be modified to fire medium-range ballistic missiles.

"However, there is no confirmed information yet that a North Korean submarine capable of launching ballistic missiles is in operation," Kim stressed.

North Korea's small submarine fleet is comprised of largely obsolete Soviet-era and modified Chinese vessels.

The US-Korea Institute at Johns Hopkins University said in June that North Korea appeared to have acquired a sea-based copy of a Russian cruise missile.

Arms control expert Jeffrey Lewis of the US think-tank said the missile would mark "a new and potentially destabilising addition" to North Korea's military arsenal.

He identified the weapon as a copy of the Russian-produced KH-35 -- a sea-skimming anti-ship cruise missile developed during the 1980s and 90s.