PARIS, Oct 11: More than two days after North Korea claimed to have exploded a nuclear weapon, scientists on Wednesday edged towards the conclusion that the test -- if indeed nuclear -- was a failure.

But a failure is unlikely to derail North Korea's nuclear programme and could even help Pyongyang's bomb makers, they warned.

Two main sources of scientific evidence are needed to confirm that a blast is nuclear.

One is the shockwave sent back by ground detectors, and the other is fallout -- radioactive particles or gases -- that often escape from an underground test site, even if the tunnel or shaft is sealed tight.

But North Korea's blast was so tiny that the seismic wave is almost indistinguishable from routine subterranean background noise, experts say.

That means it will take a long time, harnessing supercomputers and the minds of top physicists, to root out any telltale spikes that confirm the blast was nuclear -- and not, say, a stockpile of TNT blown up as a hoax.

"There is a series of differentiations to be done" to sift out the blast from background noise, said Xavier Clement, spokesman of France's Atomic Energy Commission (CEA).

"It is possible that this cannot be done, given the weakness of the signals compared to the background noise," he told AFP.

In the absence, so far, of any known radiological evidence, scientists also note the very low size of North Korea's explosion.

Only Russia has described the blast as a full-fledged nuclear event, equivalent to five to 15 kilotons (5,000-15,000 tons) of TNT, while the Norwegian institute of seismology Norsar described it as a "medium-sized bomb" at between one and 10 kilotons. But other national monitors put it at less than one kiloton, with one figure of as little as 200 tons.

"The easiest size of weapon to build is 10 to 20 kilotons. It's harder to build one that's smaller, and it's harder to build one that's larger," said James Acton of Vertic, an independent British watchdog.

"It seems to me technically unlikely and politically unlikely that North Korea would have tried to do anything other in its first test than in the 10-20 kiloton range," Acton, a Cambridge-educated nuclear physicist, said in an interview from London.

"If it was a nuclear test and the yield was less than a kiloton, it seems to me the evidence is that this test was a fizzle, it was a partially unsuccessful test."—AFP