SEOUL (AFP) - Nuclear experts cast doubt on Wednesday (Jan 6) on North Korea's claimed first successful test of a miniaturised hydrogen bomb, saying the detected seismic activity suggested a less powerful device.

The announcement followed hints in December by leader Kim Jong Un that Pyongyang had already developed a hydrogen, or thermonuclear, bomb - a claim greeted with scepticism by experts at the time.

Mr Crispin Rovere, an Australia-based nuclear policy and arms control specialist, said the 5.1 magnitude tremor detected at the North's Punggye-ri nuclear test site was too small to support Pyongyang's claim.

"The seismic data that's been received indicates that the explosion is probably significantly below what one would expect from an H-bomb test," Mr Rovere told AFP.

"So initially it seems to be that they've successfully conducted a nuclear test but unsuccessfully completed the second-stage hydrogen explosion," he said.

The test came just two days before Mr Kim Jong Un's birthday. Analysts said the North's leader had been looking for a major achievement to highlight at a rare ruling party congress scheduled for May - the first gathering of its kind for 35 years.

"I don't think it was a hydrogen bomb test. The explosion had to be larger if it was a hydrogen bomb test," said Dr Choi Kang, vice-president of the Seoul-based Asan Institute for Policy Studies.

"I think they are disguising it as a hydrogen test because Kim Jong Un mentioned it before," Dr Choi said.

A thermonuclear bomb uses fusion in a chain reaction that results in a far more powerful explosion than the fission blast generated by uranium or plutonium alone.

The North has made many unverifiable claims about its nuclear weapons strength, including the ability to strike the US mainland, which most experts dismiss - at least for now.

In September, however, the Washington-based Institute for Science and International Security had raised a red flag over what appeared to be a new "hot cell" facility under construction at the North's main Yongbyon nuclear complex.

Analysts at the think-tank said satellite images suggested it could be an isotope separation facility capable of producing tritium - a key component in the design of thermonuclear weapons.

Dr Bruce Bennett, a senior defence analyst with the Rand Corporation, was also unconvinced by the H-bomb test claim.

"If it were a real H-bomb, the Richter Scale reading should have been about a hundred times more powerful than what we saw, which would have been in the range of seven or so," he told AFP.

Dr Bennett assessed Wednesday's (Jan 6) explosion as in the 10-15 kilotonne range, just less than the Hiroshima blast in 1945.

He said the fusion element of the explosion may have failed entirely, or the fission element did not operate correctly.

But he said the increasing power of the blasts heightened the prospect of triggering an earthquake and the release of radiation from the underground test site, a source of great concern to Chinese people across the border.

Asked about next steps, Dr Bennett said: "We have to be concerned because he has this separate party congress that he's planning to do in May, which is a huge political deal.

"And he's done a test now which most people and most experts in the world will say didn't work. Is he going to be forced before May to do another test to demonstrate that they can get it to work? And that's the ultimate instability."

South Korea's National Intelligence Service, which briefed lawmakers after the North's announcement, also said it was unlikely to have been a hydrogen bomb.

A member of the parliamentary intelligence committee who attended the briefing said the NIS had seen no tell-tale signs of an explosion powerful enough to be attributed to an H-bomb.

The North's first two nuclear tests in 2006 and 2009 were of plutonium devices, while the third in 2013 was believed - though not confirmed - to have used uranium as its fissile material.

Mr Seong Chai Ki, senior research fellow at the Korea Institute for Defense Analyses, said Wednesday's was more likely a boosted fission test - generally seen as a precursor to a full H-bomb detonation.

"There has been speculation that North Korea would first test its boosted fission weapon rather than going directly to a hydrogen bomb test," he told AFP.