North Korea sets off an earthshaking explosion _ and claims it was nuclear. Was it? For scientists, that was not a quick and easy question to answer.

Like earthquakes, large explosions send out shockwaves that can be detected on seismographs. Big 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.

The natural sound of the Earth, with its constant seismic activity of tectonic plates grinding together, complicates the task of trying to determine whether a smaller blast was caused by conventional explosives or a nuclear device, said Xavier Clement of France's Atomic Energy Commission.

He likened the problem to trying to "detect the violins or a flute in a symphony orchestra when you are playing the cymbals."

His agency estimated the North Korean blast at around 1 kiloton or less _ equivalent to the explosive force of 1,000 tons of TNT. For a nuclear device, that would be so weak that the French defense minister suggested that "there could have been a failure" with the North Korean reported test.

Clement said it could take days before scientists can declare with certainty whether the explosion was nuclear. And when blasts are very weak, "we could be in a situation where we cannot tell the difference between the two," he said.

The United States, Russia, China, Japan and South Korea are among the countries with equipment strong enough and close enough to monitor a North Korean test, said Russian nuclear physicist Vladimir Orlov of the Moscow-based PIR Center, a nonproliferation think-tank.

"It takes days, dozens of lab hours, to evaluate results. Now we can have only a rough estimate," he said.

The Comprehensive Nuclear Test Ban Treaty Organization, or CTBTO, has about 200 stations worldwide designed for monitoring nuclear tests as part of what it hopes will become the world's most reliable source for such tests. But until the treaty comes into force, the data are not made public, only released to governments and vetted partners.

Seismic data comes in almost immediately, and is usually passed to governments within an hour or so. Their scientists must decide what the numbers and graphs mean...

GERTZ: U.S. doubts Korean test was nuclear; Readings fall short of atomic explosion... MORE...

U.S. intelligence agencies say, based on preliminary indications, that North Korea did not produce its first nuclear blast yesterday, WASHINGTON TIMES star reporter Bill Gertz is set to report in Tuesday editions.

U.S. officials, speaking on the condition of anonymity, 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.

'There was a seismic event that registered about 4 on the Richter scale, but it still isn't clear if it was a nuclear test. You can get that kind of seismic reading from high explosives.'

The underground explosion, which Pyongyang dubbed a historic nuclear test, is thought to have been the equivalent of several hundred tons of TNT, far short of the several thousand tons of TNT, or kilotons, that are signs of a nuclear blast, the official said. Developing...

...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, scientists told the Associated Press.

"It takes days, dozens of lab hours, to evaluate results. Now we can have only a rough estimate," said Russian nuclear physicist Vladimir Orlov of the Moscow-based Center for Policy Studies in Russia, a nonproliferation think tank.

Elements of the blast were detected by U.S. and allied sensors as it was set off in an underground tunnel in the north-central part of North Korea. U.S. intelligence agencies have been monitoring several tunnels thought to be nuclear test facilities and have not ruled out Pyongyang's conducting another test.

U.S. officials said the test was timed to coincide with several anniversaries in North Korea, including the end of mourning for the death of North Korean leader Kim Jong-il's father, Kim Il-sung. The test was thought to have been linked to the commemoration.

North Korea's military thinks that joining the world's seven other acknowledged nuclear powers is key to preserving the power of the communist regime.

There were wide variations in seismic data of the North Korean blast. The French atomic agency estimated about 1 kiloton, and South Korea's geological institute said half of that. But Russia's defense minister expressed "no doubt" that North Korea detonated a nuclear test and said the force of the underground blast was equivalent to 5,000 to 15,000 tons of TNT.

"People have different ways of cross-cutting the data and interpreting them," said Lassina Zerbo, director of the International Data Center at the nuclear-test-ban preparatory commission, which is based in Vienna, Austria...

China has reportedly cancelled all leave for troops along part of its border with North Korea.

A mainland controlled Hong Kong newspaper says China has cancelled leave for troops posted along part of the North Korean border.

The Wen Wei Po newspaper says Chinese People's Liberation Army (PLA) troops ranged along the border in the north-eastern Jilin province have had leave "totally cancelled".

It also says some PLA forces are also conducting chemical weapons training, North Korea is thought to possess chemical weapons.

Chinese media has also reported that President Hu Jintao last night had a phone conversation with US President George W Bush over North Korea's apparent nuclear test.

President Hu reportedly said China "firmly" wants a non-nuclear Korean Peninsula and opposes the proliferation of nuclear weapons.

It wants the issue resolved peacefully through dialogue and consultation. But President Hu also says Beijing still sees room for negotiations to end Pyongyang's nuclear arms

Chinese state media reports the US considers that the issue can still be resolved peacefully through diplomatic channels and will continue to communicate with the Chinese side "to this effect".

China's Foreign Minister also held talks last night with his counterparts in Britain and Japan.

The reports come a day after China released a statement condemning its neighbour's apparent nuclear test.

It called it "brazen" or "flagrant", a term only used in the past to describe enemies of China...