The White House says any nuclear test conducted by North Korea should be considered a provocative act. The comments came just a few hours after North Korea claimed it had conducted its first underground test of a nuclear weapon.
The first reaction from the White House came from spokesman Tony Snow.
In an unusual middle-of-the-night conference call with news agencies, Snow said American and South Korean intelligence agencies have detected seismic activity near a suspected nuclear test site in North Korea.
Speaking a few hours after Pyongyang claimed that it had conducted such a test, Snow made clear the United States and its allies are still monitoring the situation and gathering information.
But at the same time, he stressed any nuclear test by North Korea would be considered a provocative act in defiance of the international community and U.S. calls on Pyongyang to refrain from actions that would aggravate tensions in the region. Snow said the United States expects immediate action in the Security Council to respond to - what he called - "this unprovoked act."
The Security Council issued a stern warning to North Korea just on Friday to refrain from nuclear testing, after Pyongyang announced it was planning to take such action.
Snow said the North Koreans had called officials in China early Monday morning to tell them a test would be coming. Shortly after that, the message was passed on to the United States.
Snow declined to speculate on a possible U.S. response to a North Korean nuclear test and would not answer questions about possible military action. He said only that the United States reaffirms its commitment to protect its allies in the region - South Korea and Japan - while emphasizing that the focus now should be on international action through the U.N.
Snow went on to say that there are still many unanswered questions, and plenty of data to be assessed to confirm a test. He said much would depend on what that data reveal.