The United States confirmed Monday that North Korea conducted a nuclear test on Oct. 9, 2006.
"Analysis of air samples collected on October 11, 2006 detected radioactive debris which confirms that North Korea conducted an underground nuclear explosion in the vicinity of P'unggye on October 9, 2006," said a short press statement released by U.S. National Intelligence Director John Negroponte's office.
"The explosion yield was less than a kiloton," the statement said. Each kiloton is equal to the force produced by 1,000 tons of TNT.
The statement provides the first official confirmation from the United States that a nuclear detonation took place, as Pyongyang has claimed.
By comparison, one estimate places the Hiroshima bomb close to a 13-kiloton blast, while other estimates put it as high as 20. The Nagasaki blast is considered to be in the same vicinity.
On Friday, a senior Bush administration official told The Associated Press that one test conducted on samples gathered after the detonation found a radioactive gas consistent with a nuclear explosion. At the time, however, U.S. intelligence was not ready to confirm that a nuclear test actually had taken place.
U.S. intelligence has been poring over data collected since the explosion — air samples, seismic readings, satellite imagery and communications intercepts — in an effort to reach a conclusion on the nature of the test.
A key clue came from air samples collected by the Air Force's WC-135 Constant Phoenix, a jet designed to collect particles and gases in the air after the nuclear test. Samples are rushed back to labs in the United States for study before they loose their radioactive properties.
The first reading on Tuesday was negative, but a test on a second sample collected Wednesday was positive, according to a U.S. government intelligence official, who spoke on condition of anonymity because of the sensitive situation with Pyongyang.
The official said that the North Korean device was believed to be roughly the equivalent of 200 tons (181 metric tons) of TNT, suggesting to analysts that it was probably a partial failure. Experts in and out of government had expected a detonation of at least several thousand tons.
The findings from U.S. intelligence agencies come at a diplomatically sensitive time.
Citing new inspections by the Chinese of trucks bound for North Korea, the Bush administration said earlier Monday that it expected Beijing would do its part in enforcing a U.N. resolution punishing its reclusive ally for its nuclear program.
This came as Secretary of State Condoleezza Rice prepares to visit the region this week.
The United States would not be surprised if the North Koreans were to attempt a second nuclear test sometime soon, said the U.S. government intelligence official and two others, who also spoke on condition of anonymity.
The officials said the decision to test is considered a political one, and North Korea will be closely watching action at the United Nations and elsewhere.
The Associated Press contributed to this report.