Recent satellite images offer fresh evidence of North Korea developing a marine-based missile system that would give the nuclear-armed state a survivable second-strike nuclear capability, a US think-tank said Friday.

The commercial satellite pictures suggest the conning tower of a new North Korean submarine -- first seen in July last year -- houses one or two vertical launch tubes for either ballistic or cruise missiles, the US-Korea Institute at Johns Hopkins University said.

"The boat could serve as an experimental test bed for land-attack missile technology which, if successful, may be integrated into a new class of submarines," the institute said in an analysis posted on its closely-followed 38 North website.

Development of a submarine-launched missile capability would take the North Korean nuclear threat to a new level, allowing deployment far beyond the Korean peninsula.

However, the institute noted Pyongyang possessed no such capability as yet and stressed that its development would be an extremely "expensive and time-consuming endeavour" with no guarantee of success.

North Korea's small submarine fleet is comprised of largely obsolete Soviet-era and modified Chinese vessels, but suggestions that it is experimenting with a marine-based missile system have been around for a while.

The South Korean Defence Ministry cited intelligence reports in September that Pyongyang was understood to be developing a vertical missile launch tube for submarine use.

Ministry spokesman Kim Min-Seok said the North's 3,000-ton Golf-class submarine could be modified to fire medium-range ballistic missiles.

And in October, a separate satellite image analysis by the US-Korea Institute identified a new missile test stand at the Sinpo South Shipyard in northeastern North Korea.

The size and design of the stand suggested it was intended to explore the possibility of launching ballistic missiles from submarines or a surface naval vessel, the institute said.

While submarines carrying ballistic missiles could provide the North Korea with a survivable second-strike nuclear capability, the institute again stressed that Pyongyang was likely "years" from achieving the required technology.

Although there is no doubt that North Korea has an extremely active ballistic missile development programme, expert opinion is split on how much progress it has made.

In 2012, Pyongyang demonstrated its rocket capabilities by sending a satellite into orbit, but it has yet to conduct a test that would show it had mastered the re-entry technology required for an inter-continental ballistic missile (ICBM).

The other key question is how close the North is to being able to miniaturise a nuclear device that could be fitted on the tip of a missile.

In a white paper published earlier this month, the South Korean defence ministry said the North had already taken its miniaturisation technology to a "significant" level.

A US Defense Intelligence Agency report leaked in 2013 reached the same conclusion, although US officials at the time said it did not represent a consensus view of all the country's spy agencies.