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 explosion. 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.

The statement comes at a diplomatically sensitive time.

The Bush administration, citing new inspections by the Chinese of trucks bound for North Korea, said Monday it expected China would do its part in enforcing a U.N. resolution punishing its reclusive ally for its nuclear program.

The United States is pressing China for tough action against North Korea ahead of Secretary of State Condoleezza Rice's trip this week to Asia.

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 against North Korea for that country's reported nuclear test.

"China signed up for this resolution and under U.N. law it's mandatory. It's Chapter Seven. The resolution was unanimous. "I talked to the Japanese and Australian governments. Both of those governments are beginning to stop shipments," Burns told FOX News on Monday. "The point of all of this so to isolate the North Koreans, to raise the cost to them and frankly to make these sanctions hurt."

Burns said Rice was traveling to Beijing this week and was going to stress to the leadership that its ambassador's comments that China would not follow would be acceptable.

"It's not often when you vote for a resolution that says a country must do something you say you are not going to do it. We suspect that China will reconsider that situation or perhaps the
ambassador misspoke," Burns said.

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?"

China, which voted Saturday for the U.N. penalties, has balked at cargo inspections to prevent trafficking of certain banned weapons and technology.

"I'm quite certain that China is going to live up to its responsibilities," Rice said Sunday, adding she was willing to have "conversations" during her trip on how best to enforce the resolution.

The United States' U.N. ambassador portrayed North Korea's detonation last week as a public humiliation for China, which shares a long border with North Korea and is the North's chief ally and supplier of crucial shipments of food and energy aid.

If China were to cut its support, John Bolton said, it "would be powerfully persuasive in Pyongyang," the North's capital. "They've not yet been willing to do it. I think that China has a heavy responsibility here."

Rice, who joined Bolton in making the rounds of the Sunday talk shows, plans to visit Asian partners this week to consult about the resolution. "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," Rice said.

Bolton said the United States' concept of the resolution "is that the overwhelming predominance of the inspections would take place in ports or at land crossings or that sort of thing. But the resolution neither increases nor decreases existing authority to interdict on the high seas."

The U.S.-sponsored resolution demands 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.

"It is so important not to allow this to become a bilateral negotiation, because the North would like nothing better than to simply deal with the United States so that we are the ones that isolate it," Rice said.

Critics said the U.N. penalties will not curb North Korea's nuclear ambitions and stemmed from what they saw as President Bush's failed foreign policy.

Democratic Sen. John Kerry said the Bush administration is "living in a complete fantasy with respect to the foreign policy they put in place. It is a failure." He said U.S. involvement in Iraq has undermined America's credibility to deal with nuclear threats in North Korea and Iran.