The United States confirmed for the first time today that North Korea did indeed carry out a nuclear test last week.

A statement from the office of John Negroponte, the US National Intelligence Director, said that samples taken from above the test site in North Korea had picked up radioactivity linked to a nuclear detonation.

But it said the blast had produced an explosive yield of under one kiloton - which, according to independent experts, means that it was probably a technical failure.

The analysis is the first official confirmation that the North Koreans did test a nuclear device last Monday after both Chinese and South Korean scientists failed to find any evidence of radioactivity,

Although Russian scientists claimed within hours of the test that they were 100-per cent certain the state had detonated a weapon measured at between five and 15 kilotons, the relatively modest seismic impact convinced France and the US to rate it as under a kiloton.

South Korea suggested the yield had been as little as 550 tons and some experts speculated the test was a fraud with conventional explosive used to simulate the effect of a nuclear explosion.

The US results indicate the North Koreans did make a genuine attempt to explode a nuclear weapon but failed.

The statement from the Office of the Director of National Intelligency said: "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 explosion yield was less than a kiloton."

James Acton, of the London-based Verification Research, Training and Information Centre (Vertic), said that most first-generation nuclear devices are between 10 and 20 kilotons because it is the easiest size to build. The bomb dropped on Hiroshima measured 12.5 kilotons.

He said: "Both from a technological as well as a political analysis, you would expect North Korea to build a 10-20 kiloton bomb. The fact that it is smaller than that suggests the test was not very successful."

The US statement came as China’s UN Ambassador, Wang Guangya, said that his country would back the sanctions imposed by the organisation by carrying out inspections but indicated it would not approve of any attempt to impose a naval blockade on the state.

China voted on Saturday for the UN Security Council resolutions that imposed sanctions on North Korea for its nuclear test but Mr Wang said the resolution did not make it mandatory for all nations to inspect cargo going to and from North Korea.

Although Chinese customs inspectors have begun inspecting cargo trucks bound for North Korea in the border city of Dandong, officials within the Bush Administration said that the United States expected Beijing would do its part in enforcing the UN resolution. Condoleezza Rice, the US Secretary of State, is due to fly to the region later this week to reinforce the position.

The US-sponsored resolution demands that North Korea eliminate nuclear weapons but rules out military action against the country. It bans countries from exporting any weapons or material that could be used in missile programmes as well as targeting the luxury goods enjoyed by senior members of Kim Jong Il's regime.

Nicholas Burns, Under-Secretary of State for Political Affairs, said: "We have indications this morning the Chinese are beginning to apply this to their very long land border with North Korea."

He added that 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," he said.

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