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

WASHINGTON -- The United States and South Korea detected a seismic event Sunday night at a suspected nuclear test site in North Korea, the White House said Monday after North Korea claimed it had conducted an underground nuclear test.

"At this point we are not confirming a nuclear test," White House spokesman Tony Snow emphasized.

"A North Korean nuclear test would constitute a provocative act in defiance of the will of the international commuity and of our call to refrain from actions that would aggravate tensions in Northeast Asia," Snow said.

"We expect the U.N. 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 10 p.m. EDT Sunday. South Korea's Yonhap news agency reported that the suspected test was conducted at 9:36 p.m. EDT Sunday.

Snow declined to speculate on a possible U.S. 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 U.S. government official, who spoke on condition of anonymity also because of political sensitivity of the situation, 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 Sunday's apparent action, Pyongyang had never tested a device.

U.S. 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 last week, U.S. officials have been anticipating news of a nuclear weapons test in North Korea.

"It would be a very provocative act by the North Koreans," Secretary of State Condoleezza Rice said Tuesday. "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."