White House Confirms 'Seismic Event' Near Suspected N. Korean Nuclear Test Site

Assistant Secretary of State Christopher Hill, the top U.S. negotiator on the communist country's nuclear program, said 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."

Defense Secretary Donald H. Rumsfeld said Thursday that 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 Tuesday that a North Korean test "would be contrary to the interests of all of North Korea's neighbors and to peace and security in the Asia-Pacific region."

In recent weeks, U.S. 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 U.S. military, the organization 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 U.N. 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 U.S. and its allies have been trying to lure North Korea back to stalled international efforts to persuade Pyongyang to scrap its nuclear weapons program.

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 U.S. financial restrictions imposed for its alleged illegal activity, including money laundering and counterfeiting.