WASHINGTON – Air samples gathered last week contain radioactive materials that confirm that North Korea conducted an underground nuclear explosion, National Intelligence Director John Negroponte’s office said Monday.

In a short statement posted on its Web site, Negroponte’s office also confirmed that the size of the explosion was less than 1 kiloton, a comparatively small nuclear detonation. Each kiloton is equal to the force produced by 1,000 tons of TNT.

“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,” the statement said.

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.

The statement from Negroponte’s office provides the first official confirmation from the United States that a nuclear detonation took place, as Pyongyang has claimed.

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 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.

R. Nicholas Burns, undersecretary of state for political affairs, pointed to the fact that Chinese customs inspectors had begun inspecting cargo trucks bound for North Korea in the border city of Dandong. Japan and Australia, meanwhile, announced Monday that they might take measures beyond the new U.N. sanctions.

“We have indications this morning the Chinese are beginning to apply this to their very long land border” with North Korea, Burns said on CBS’ “The Early Show.” “We also have announcements from Japan and Australia. I spoke to both governments this morning. They are both implementing this.”

He said there will be “enormous pressure on China to live up to their responsibility” in enforcing United Nations punishment of its ally, North Korea. “We are all banking on that.”

White House press secretary Tony Snow said President Bush had not personally been making any calls Monday on the matter. Snow urged patience before judging China’s commitment to the inspections.

“The parties have committed to fulfilling its conditions,” Snow said. “Let’s see what happens, all right?”

Rice, who joined U.N. Ambassador John Bolton in making the rounds of the Sunday talk shows, plans a series of talks aimed at easing tensions among countries already on edge because of the test. “I understand that people are concerned about how it might work so it doesn’t enhance tensions in the region, and we’re perfectly willing to have those conversations,” she said.

The United States would not be surprised if North Korea 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 U.S.-sponsored U.N. resolution demands that North Korea eliminate nuclear weapons but rules out military action against the country, as the Russians and Chinese demanded.

After the resolution unanimously passed, North Korea’s U.N. ambassador accused council members of a “gangster-like” action that neglects the nuclear threat posed by the United States.

Meanwhile, a leading Senate Republican urged direct talks with North Korea, as the reclusive nation has sought. “We do need to engage the North Koreans” because the U.N. resolution is weak and limited, said Sen. Chuck Hagel of Nebraska, the second-ranking Republican on the Senate Foreign Relations Committee.

But Rice brushed aside such calls, reaffirming the U.S. commitment to six-nation disarmament talks, which have stalled.