North Korea carried out a third submarine-launched ballistic missile test last month, reports said Wednesday, as US researchers warned of the "significant threat" such a capability, if fully developed, would pose.

The South Korean and US media reports, citing military officials in both countries, said the test appeared to have been conducted from a submerged submarine near the port city of Sinpo.

It was unclear if it was meant to be a simple ejection test or a full flight test, the reports said, making it difficult to determine the success or failure of the exercise.

Both the South Korean defence ministry and the Pentagon declined to comment on what they said was an intelligence matter.

Researchers with the US-Korea Institute at Johns Hopkins University said recent satellite images of the naval shipyard at Sinpo showed stepped-up activity that supported the reports of a third test being carried out.

The images also suggested a submarine reported to have been damaged in a failed test in November was still "seaworthy".

An ongoing refurbishment project at the Sinpo South Shipyard was seen as nearing completion -- allowing for the construction of a new, much larger class of submarines than those in the current North Korean fleet, the researchers said.

They concluded that North Korea was continuing to "actively pursue" the development of a submarine-launched ballistic missile (SLBM) and associated ballistic missile submarine, with the combined "potential to present a significant threat in the future".

However, they added that the development of an operational system would be an "expensive, time-consuming endeavour" with no guarantee of success.

North Korea has a 70-strong submarine fleet -- one of the largest in the world -- but it is comprised of largely obsolete Soviet-era and modified Chinese vessels incapable of handling an operational SLBM system.

A fully developed SLBM capability would take the North Korean nuclear threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.

North Korea first announced in May that it had conducted a successful SLBM test -- a claim accompanied by pictures of leader Kim Jong-Un pointing at the missile as it blasted out of the water.

Numerous experts suggested the images were doctored and the test was widely assumed to have been a simple ejection test, rather than a full flight test.

It was also believed to have been launched from an underwater platform, and not a submarine.

North Korea regularly makes claims of major advances in its military capabilities -- particularly in connection with its nuclear arsenal.

Last year it boasted of having mastered the technology to place a miniaturised nuclear warhead of a ballistic missile and, in December, Kim suggested it had already developed an advanced thermonuclear device.

Although expert opinion differs on what stage the North has reached in its nuclear and missile development programmes, there is unanimity that it has made substantial strides on both fronts in recent years.