Washington - US and South Korean intelligence detected a seismic event at a suspected nuclear test site in North Korea, the White House said Monday, a few hours after North Korea claimed it had conducted an underground nuclear test.

"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," White House spokesman Tony 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 US President Bush about the event shortly before 10 pm EDT Sunday. South Korea's Yonhap news agency reported that the test was conducted at 9:36 p.m. EDT Sunday.

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."