The official said the North Koreans could test again with little notice but said activity at potential test sites were not necessarily an indication of preparations for another test.

The office of the Director of National Intelligence said the confirmatory air samples were collected on October 11, two days after North Korea declared that it had detonated a nuclear weapon for the first time in a test.

"Analysis of air samples collected on October 11, 2006 detected radioactive debris which confirms that North Korea conducted an underground nuclear explosion in the vicinity of P'unggye on October 9, 2006," the office of the Director of National Intelligence said.

"The explosion yield was less than a kiloton," the statement said.

Analysis of air samples collected by a US military aircraft on October 10 showed no evidence of nuclear debris, a defense official told AFP.

But radioactive debris showed up in a second batch of air samples collected the next day, said the official, who asked not to be identified.

The official said the presence of xenon gas was among the tell-tale signs of a nuclear explosion.

Nuclear explosions emit xenon and krypton gases that eventually will seep through rock and soil into the atmosphere, analysts said.

The unusually low yield, estimated by US intelligence officials at as low as the equivalent of 200 tons of TNT, had raised doubts that North Korea had succeeded in detonating a nuclear device as claimed.

North Korea is reported to have tipped off China before the test that it was detonating a four kiloton weapon.

The DNI would not comment beyond its statement so it was unclear whether the US intelligence community believes the North Korean test was successful or not.

"Those are the kinds of questions that are being looked at hard," said the US official.

Although the working assumption is that the North Korean device failed to meet specifications, analysts have not ruled out that the blast was muted by the geology of the test site.

Another possibility is that North Korea deliberately tested a sub-kiloton weapon, but that would require far greater sophistication than would be expected from a country that had never tested a nuclear weapon before.

Robert Norris, a nuclear weapons expert at the Natural Resources Defense Council, said a partial yield may have resulted from the misfiring of high explosive charges used to trigger a plutonium bomb.

"We're pretty sure that this is an implosion design which means you're pressing the plutonium ball inward with conventional high explosives," he told AFP.

The triggering mechanism involves detonating 32 or 64 precisely shaped high explosive charges at exactly the same time to drive pressure inward on the plutonium core, he said.

"Otherwise, if it doesn't there won't be uniform compression of the ball of plutonium, and you'll only get a partial yield. If it's so bad, it probably wouldn't go off at all," he said.

In this case, he said, the high explosive charges might have been directed incorrectly or they might not have gone off at the same time.

"No doubt the North Koreans are poring over whatever data they were able to gain from this test and they'll learn something from this, which means there might be another test at some future point correcting the problems that they found in this one," he said.

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