U.S. spy satellites have detected what may be preparations for North Korea's first test of a nuclear weapon, although analysts believe it could be a calculated ruse on Pyongyang's part, a U.S. defense official said Friday. The satellite images show North Korea has dug and refilled a significant hole at a suspected test site in Gilju in the northeastern part of the country, said the official, discussing intelligence only on the condition of anonymity. The hole was dug in a manner consistent with preparations for an underground nuclear test, although it is not known whether the North Koreans deposited a weapon inside, the official said. In addition, the official said, they have built some bleachers a sufficient distance from the hole, presumably for viewing any test. Officials elsewhere in the U.S. government played down the remarks. One who spoke on condition of anonymity said activity at the site could be consistent with preparations for a nuclear test, but other explanations are also possible. The official said the U.S. government's working assumption is that North Korea could test with little notice and is believed to have the technical capability to do so. Its decision is considered to be one of politics. Presidential spokesman Scott McClellan said he didn't want to get into discussing intelligence matters. "But what I would say is that if North Korea did take such a step, that would just be another provocative act that would further isolate it from the international community." Mohamed ElBaradei, director general of the International Atomic Energy Agency, said a nuclear test by North Korea would be "a grave mistake." "It would have grave implications, security implications, political implications, possibly environmental implications in terms of radiological fallout," he said in an interview with the Associated Press. "I think the message should be very clear to the North Koreans that nuclear blackmail does not work and that, whatever the settlement, they need to roll back their nuclear weapon program, that the international community in 2005 has zero tolerance for any new nuclear weapon states." The six-nation talks aimed at getting Pyongyang to forego its nuclear ambitions have been stalled for nearly a year. They involve North and South Korea, the United States, China, Japan and Russia. North Korea has boycotted the talks since June, and on Friday reaffirmed it would stay away unless the United States dropped what it called hostile policy toward the communist regime.