An air sampling taken after North Korea's claimed nuclear test detected radioactive debris consistent with an atomic explosion, Bush administration and congressional officials said. They said no final determination had been made about the nature of last weekend's mystery-shrouded blast.
One U.S. government official said intelligence officials assigned an 80 percent probability that the North Korean explosion was a nuclear detonation, based on the air sample collected Wednesday. The official said it appeared highly unlikely that the sample of radioactive material was produced by any other source, including a nuclear power reactor.
The official also said additional sampling might be conducted, not necessarily by airborne means. He would not elaborate, citing security concerns.
A senior administration official suggested that the North Korean test was a dud. "The betting is that this was an attempt at a nuclear test that failed," the official said. "We don't think they were trying to fake a nuclear test, but it may have been a nuclear fizzle."
The officials who described the results spoke Friday night on condition of anonymity because of the sensitive nature of the information.
North Korea's claim of a successful nuclear test Monday sent shock waves throughout Asia and around the world. President Bush has called for stiff United Nations sanctions on North Korea, while refusing appeals by U.N. Secretary-General Kofi Annan and others to take part in one-one-one talks with the reclusive communist regime.
Since North Korea's announcement, the United States and other nations have been conducting scientific tests to determine whether a nuclear explosion had occurred.
The administration briefed key members of Congress about the preliminary test results of an air sample an official said was collected above Qunggye, near the area of the claimed nuclear test.
Results from another test disclosed Friday — an initial air sampling on Tuesday — showed no evidence of radioactive particles that would be expected from a successful nuclear detonation, a U.S. government intelligence official said.
The contradictory readings reinforced uncertainty about the size and success of Monday's underground explosion, which North Korea has trumpeted as a nuclear test. Data from seismic sensors have already indicated the explosion was smaller than expected.
The Chinese and Japanese governments have done their own air sampling and found no trace of radioactive material, officials from both countries said Friday. A Japanese government official said his country sampled air over the Sea of Japan, as well as rainfall and ground-level air on Japanese territory and found nothing.
A spokesman for National Intelligence Director John Negroponte declined to comment on any findings from U.S. spy agencies.
The State Department, meanwhile, announced that Secretary of State Condoleezza Rice will travel to China, South Korea and Japan next week to discuss steps to pressure North Korea to drop its nuclear efforts and to assess the region's security situation.
Members of the U.N. Security Council agreed Friday on wording of a resolution that would clamp sanctions on the communist country. The draft, scheduled for a Saturday vote, would authorize nonmilitary sanctions against the North, and says that any further action the council might want to take would require another U.N. resolution.
It also eliminates a blanket arms embargo from a tougher, previous draft, instead targeting specific equipment for sanctions including missiles, tanks, warships and combat aircraft.