France's Defence Minister Michele Alliot-Marie leaves a hotel in Paris in this September 27, 2006 photo. She said North Korea reported nuclear bomb test produced an explosion so small that if indeed nuclear, it was a failure. [Reuters]

French experts have not yet determined the precise cause of the DPRK's explosion, she told French radio, but French, US and other scientists detected that it was of "relatively limited size."

"In any case, if this was a nuclear explosion, it would be a case of a failed explosion," she said, adding "that doesn't change anything about the severity of things" because it would still hurt efforts to bring stability to East Asia.

French experts, working from international seismological data gathered at the time of the blast, say the explosion mustered a force equivalent to that caused by 500-1,000 tons of TNT.

However, Russian Defence Minister Sergei Ivanov initially said the blast had a destructive power of between 5,000-15,000 tons of TNT. The atomic bomb that struck Hiroshima, Japan, on August 6, 1945, also had a force of about 15,000 tons.

Xavier Clement of France's Atomic Energy Commission said he could not explain how the Russians could have come up with that figure.

"It's a hypothesis, not shared by us and other countries who also possess this type of high-precision analysis," he said in a telephone interview. "(The information) we are giving right now is credible."

Clement said the Comprehensive Nuclear Test Ban Treaty Organization, which has some 200 stations worldwide that can monitor nuclear tests, has not yet provided member country France with any radiological data that could help determine the type of explosion.

"What's important is to determine is whether this was chemical or nuclear," Clement said, saying that seismological data was being collected to try to differentiate between the two. "It's possible that we never will."

Alliot-Marie, who relies on the atomic commission for her information on the test, on Monday was among the first Western policy makers to suggest it could have been a dud.

The comments yesterday were her strongest so far about the possibility of a failed test, suggesting that that theory was gaining credence among French experts.

Although the blast from the nuclear weapons test was strong enough to send seismic waves as far as Japan's main island, verifying exactly what happened could take several more days, if not weeks, officials said yesterday.

"We are still currently investigating various matters and we will continue to work on confirming the facts," said Japan's chief government spokesman, Yasuhisa Shiozaki. Shiozaki has said officials are not even at the stage when they can predict how long the verification process will take.

US military officials, meanwhile, remained quiet, confirming only that they are monitoring the situation. The United States has WC-135 reconnaissance planes deployed on the southern Japan island of Okinawa that can sniff out radiation in the air and glean other data on the test.

Washington also regularly flies U-2 spy planes out of an air base in the Republic of Korea (ROK).

Yesterday, a British Royal Air Force VC-10 refuelling plane arrived at Yokota Air Base, just outside of Tokyo, to assist in the verification effort under the auspices of the United Nations. It later flew to Kadena Air Base on Okinawa, where the WC-135s are based.

"The UK is assisting in international operations in respect to verifying the DPRK's nuclear tests," said Clive Hughes, the press attache for the British Embassy in Tokyo. He refused to provide further details.

Though it would seem easy to confirm a nuclear explosion, the technical challenges can be daunting.

The White House has acknowledged there is a "remote possibility" that the world never will be able to fully determine whether the DPRK succeeded in conducting the test.

A leading Japanese nuclear expert agreed.

"It is possible with a relatively small underground blast that there would be no significant release of radiation," said Hideshi Takesada, of the National Institute for Defence Studies, a branch of Japan's Defence Agency.