North Korea's "successful" submarine-launched ballistic missile test last month was, in fact, an explosive failure that was not even launched from a submarine, separate expert analyses concluded Wednesday.

The North released a video of the purported December 21 test on Saturday, showing leader Kim Jong-Un -- dressed in a winter coat and fedora hat -- looking on as a missile was launched vertically from underwater and ignited in mid air.

The video then cut to a rocket flying through the clouds.

The footage was almost immediately dismissed as a fake -- digitally manipulated footage of the actual launch and spliced footage from other missile tests to give the impression of a successful outcome.

Analysts at the California-based James Martin Center for Nonproliferation Studies said the missile almost certainly blew up after a successful pop-up.

"Although (it) appears to eject successfully... we think that a catastrophic failure occurred at ignition," said Catherine Dill, a research associate at the centre.

North Korea "manipulated the footage in an attempt to obscure this result, but one clip plays for two frames too long. The rocket appears to explode," Dill said.

The outside world has been paying close attention to the North's efforts to acquire a submarine-launched ballistic missile (SLBM) capability which, if fully developed, would take its nuclear strike threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.

The December test was understood to be the third of its kind since May.

A second test in November was also believed to be a failure, with South Korean intelligence reports suggesting the submarine launch vehicle had been badly damaged.

In a separate analysis of the December launch video, experts at the US-Korea Institute at Johns Hopkins University noted several frames showing what appeared to be a support vessel berthed as little as 50 metres from the launch site.

"That would be dangerously close to a submarine operating at shallow depth for a missile launch. But it would be just about right, and quite necessary, for a submerged barge," aerospace engineer John Schilling wrote on the institute's website 38 North.

"So the test was most likely from a barge, not a submarine," he added.

Whether the test was successful or not, Schilling said it was clear that the North Koreans were intent on developing an SLBM capability and "will presumably get it right eventually".

However, he predicted that developing a fully operational SLBM system would take until 2020.