North Korean state television aired footage on Friday of the latest test, said to have taken place in December. Unlike a previous SLBM test in May, it had not been announced at the time.

"The rocket ejected, began to light, and then failed catastrophically," said Melissa Hanham, a senior research associate at the California-based Middlebury Institute's James Martin Centre for Nonproliferation Studies (CNS).

South Korea's military said on Saturday North Korea appeared to have modified the video and edited it with Scud missile footage from 2014 although an official told Reuters that the ejection technology might have improved since the May test.

The CNS analysis shows two frames of video from state media where flames engulf the missile and small parts of its body break away.

"North Korea used heavy video editing to cover over this fact," Hanham said in an email.

"They used different camera angles and editing to make it appear that the launch was several continuous launches, but played side by side you can see that it is the same event".

North Korean propagandists used rudimentary editing techniques to crop and flip old video footage of an earlier SLBM test and Scud missile launch, the video analysis showed.