Initial tests of air samples taken by U.S. planes near North Korea found no evidence of radiation, but the United States is not ready to declare that Pyongyang did not detonate a nuclear device, a U.S. government intelligence official said on Friday.

Monday's announcement by the reclusive communist state that it had tested a nuclear bomb sharply escalated world concerns over North Korea's nuclear program. But the United States and other countries have not yet confirmed whether or not an underground nuclear test was conducted, or whether conventional weapons were used.

"The initial test came back negative," the U.S. official, speaking on condition of anonymity because of the sensitivity of the issue, told Reuters of air sample testing.

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The official said further analysis was being conducted, but "I doubt it will differ from the initial one."

However, the official said U.S. intelligence experts were not ready to state that the North Korean test was non-nuclear or alternatively that it was a failed nuclear device.

The air samples were taken by U.S. Air Force planes and were later analyzed by U.S. experts for signs of radioactivity that would indicate that a nuclear explosion had occurred, the official said.

"It would be wrong of us I think to just go out and say with 100 percent clarity, 'Due to no radiation being detected, therefore our conclusion is it was TNT or conventional explosives,'" the official said.

"You've got enough differing opinions out there saying, well, it was a failed nuclear device and that's why you got the pop instead of the bang. So it's going to be tough to come out and say that," the official said.

The official said that the absence of detected radiation could mean several things, and noted that the North Koreans could argue that no radiation was detected because the nuclear test was conducted so skillfully that all the radiation was contained at the underground test site.

"Then you've got some of the scientists out there saying it's very, very difficult to contain it all," the official said.

White House spokesman Tony Snow said, "The most significant thing at this point is that the North Koreans said that they were going to detonate and announced afterward that they did detonate a nuclear device."

U.S. says Security Council agrees on sanctions vote Saturday
U.S. Ambassador John Bolton said Security Council members had agreed to vote on Saturday on a resolution imposing sanctions on North Korea for its announcement it had tested a nuclear weapon.

Bolton spoke to reporters after consultations among the 15 council nations on Friday and said there might still be some changes in the text during the course of the day.