White House confirms 'seismic event' in North Korea

“A North Korean nuclear test would constitute a provocative act in defiance of the will of the international community and of our call to refrain from actions that would aggravate tensions in north-east Asia,” Snow said.

“We expect the UN Security Council to take immediate actions to respond to this unprovoked act,” Snow said. “The United States is closely monitoring the situation and reaffirms its commitment to protect and defend our allies in the region.”

Snow said national security adviser Stephen Hadley informed President Bush about the event shortly before 10pm yesterday (3am Irish time).

Snow declined to speculate on a possible US response to a North Korean nuclear test. “At this point we’re still assessing the data and trying to figure out what happened,” he said. “A lot of this hinges on what the data tells us.”

A US government official said the seismic event could have been a nuclear explosion, but its small size was making it difficult for authorities to pin down.

In their initial assessments, analysts believe the test appeared to be “more of a fizzle than a pop,” the official said.

North Korea is believed to have enough plutonium for as few as four and as many as about a dozen nuclear bombs. But until yesterday’s apparent action, Pyongyang had never tested a device.

US intelligence has been closely watching several sites in North Korea that could be used for a nuclear test. Movements of people, automobiles, fencing and other items convinced some analysts last week that a test could come soon. Guest quarters overlooking one site were also of interest.

Over the past week, US officials have been anticipating news of a nuclear weapons test in North Korea.

“It would be a very provocative act by the North Koreans,” said Secretary of State Condoleezza Rice. “A North Korean nuclear test ... would create a qualitatively different situation on the Korean peninsula. I think that you would see that a number of states in the region would need to reassess where they are now with North Korea.”

Assistant Secretary of State Christopher Hill, the top US negotiator on the communist country’s nuclear programme, said on Thursday that if North Korea conducted a nuclear test, “We would have no choice but to act and act resolutely to make sure (North Korea) understood, and make sure every other country in the world understands, that this is a very bad mistake.”

Defence Secretary Donald Rumsfeld said a successful North Korean nuclear weapon test would show weakness on the part of the international community.

“And that failure ... is something that the international community would have to register and ask itself how comfortable are we being that ineffective in this situation,” Rumsfeld said.

White House spokeswoman Dana Perino said on Tuesday that a North Korean test “would be contrary to the interests of all of North Korea’s neighbours and to peace and security in the Asia-Pacific region.”

In recent weeks, US government experts were poring over intelligence from the highly opaque nation, including satellite imagery. Other agencies geared up to detect a possible test.

Within the US military, the organisation primarily responsible for reporting on nuclear events is the Air Force Technical Applications Center, headquartered at Florida’s Patrick Air Force Base. It operates a worldwide network of nuclear event detection sensors, which includes satellites and a ground-based seismic network.

John Pike, director of Washington-based think tank Globalsecurity.org, said in a recent interview that North Korea likely dug a tunnel through many hundreds of feet of rock to a “zero room,” where the device would be detonated.

Scientists would run cables back to the surface that would allow them to measure readings from the explosion. The tunnel would then be stemmed and sealed to try to prevent venting.

The North Koreans’ “concern about the thing not venting has very little to do with their environmental protection agency – and a great deal to do with the Americans picking up information about the bomb and how well it worked,” Pike said.

He said the United States, using equipment including a specially-equipped aircraft, would no doubt work diligently to collect as much intelligence as possible.

The UN Security Council urged North Korea on Friday to cancel the planned nuclear test and return immediately to talks on scrapping its nuclear weapons program, saying that exploding such a device would threaten international peace and security.

A statement adopted unanimously by the council expressed “deep concern” over North Korea’s announcement.

The US and its allies have been trying to lure North Korea back to stalled international efforts to persuade Pyongyang to scrap its nuclear weapons programme.

The North has pushed for direct talks with the United States, something Washington says it will not do outside the framework of the stalled six-nation talks. The North has refused to return to the disarmament talks because of US financial restrictions imposed for its alleged illegal activity, including money laundering and counterfeiting.