North Korea appears to have acquired a sea-based copy of a Russian cruise missile, the latest step in an effort to enhance its maritime strike capability, a US think-tank said Tuesday.

A state propaganda film disseminated on social media sites, including YouTube, provides a very brief glimpse of the missile being launched from a naval vessel.

Writing on the closely watched 38 North website of the US-Korea Institute at Johns Hopkins University, arms control expert Jeffrey Lewis said the missile would mark "a new and potentially destabilising addition" to North Korea's military arsenal.

Lewis 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.

Although the range and payload of the KH-35 fall below the threshold set by the Missile Technology Control Regime, any export of cruise missiles to North Korea would be a violation of UN sanctions.

"Although direct sale from Russia seems most likely, it is possible that North Korea obtained them from a third party like Myanmar," said Lewis, who is director for East Asia at the James Martin Center for Nonproliferation Studies.

As well as Myanmar, Russia has exported sea- and land-based cruise missiles to Algeria, India, Vietnam and Venezuela.

"The possibility that North Korea might sell KH-35 technology to others ... is not a happy thought," Lewis said.

The development of the North's conventional weaponry has largely been overshadowed by concerns over its nuclear weapons programmes.

Last month, 38 North published satellite photos showing two new North Korean warships -- the largest it has constructed in 25 years.

The website said the two helicopter-carrying frigates represented an "important wake-up call" about the effectiveness of sanctions.

The flip-side of the North's naval capability was shown in pictures released Monday by the official KCNA news agency, showing supreme leader Kim Jong-Un riding in the turret of a rusted Romeo-class submarine developed by the Soviets in the 1950s.

"The submarines that our Navy holds are far superior," commented South Korean Defence Ministry spokesperson Kim Min-Seok.