WASHINGTON, Oct 10: The White House said on Tuesday it was still not certain whether North Korea had conducted a nuclear test or just ‘dusted off’ an old device.

There’s a ‘remote possibility’ that the world never will be able to fully determine whether North Korea succeeded in conducting a nuclear test, said White House press secretary Tony Snow.

While acknowledging that the action was provocative, Mr Snow suggested that it's possible that the test was something less than it appeared.

"You could have something that it very old and off-the-shelf here, as well, in which case they've dusted off something that is old and dormant," he said.

He said the US intelligence community is continuing to assess the explosion.

South Korea, however, believes that North Korea had tested an actual nuclear bomb. But Unification Minister Lee Jong Seok said it would take some time to verify the test and until it is verified, South Korea will not recognise North Korea as a nuclear power.

Reports in the US media, based on unnamed intelligence sources, also questioned the authenticity of the North Korean test.

They said that seismic readings show that the conventional high explosives used to create a chain reaction in a plutonium-based device went off, but that the blast's readings were shy of a typical nuclear detonation.

A successful nuclear detonation requires a properly timed and triggered conventional blast that splits atoms, setting off the nuclear chain reaction that produces the massive explosions associated with atomic bombs.

The White House spokesman said assessing the validity of North Korea's claim of a successful nuclear test could take several days.

"We need to find out precisely what it is that took place yesterday, and that is something that's going to take awhile for the scientists and others to work through," Mr. Snow said.

"Nobody could give me with any precision how long it will take until they can say with certainty what happened."

Nuclear bombs make big waves, with clear signatures that make them fairly easy to detect, analyze and confirm that they were caused by splitting atoms. But smaller blasts -- as North Korea's appears to have been -- are trickier to break down.

Elements of the blast were detected by US and allied sensors as it was set off in an underground tunnel in the north-central part of North Korea. US intelligence agencies have been monitoring several tunnels thought to be nuclear test facilities.

Scientists are taking a wait-and-see attitude, insisting that only careful analysis of data returned by seismic or atmospheric sensors will say whether the blast was a success or a damp squib.

Agencies add: Japan said on Tuesday no unusual radiation levels had been detected in dust samples collected by its military planes after North Korea's announcement it had conducted a nuclear test.

"So far, no abnormal amount of radioactive particles has been detected," said a report submitted to a special government task force dealing with possible fallout from the alleged underground test.

The Japanese government remains sceptical as to whether North Korea did conduct a nuclear test.

"We've launched T-4 planes of the Air Self-Defence Force to monitor radioactive materials in the air," Mamoru Kotaki, press secretary of the Defence Agency, told reporters.

The planes collected dust in the skies over the major Japanese islands of Kyushu, Honshu and Hokkaido at an altitude of about 3,000 metres, the report said.

The material will be assessed for up to two weeks in a bid to verify North Korea's announcement that it had tested its first atom bomb, officials said.

"But even if they do not detect any unnatural radioactive materials, it would be hard to declare that Pyongyang's nuclear test a failure as it was presumably conducted underground," Defence Agency spokeswoman Yoko Yato said.

"It is hard to say," if there has been a successful nuclear test as North Korea claims, Japan's Prime Minister Shinzo Abe said in parliament.

The United States has also been cautious about confirming North Korea's nuclear announcement.

South Korean officials said they believed North Korea's claim was genuine, but they planned to borrow hi-tech equipment from Sweden to test the atmosphere for radioactive particles.