There has been no independent confirmation of the test

But they stressed that more tests were needed to reach a conclusion.

A vote on sanctions is expected later in the day at the UN Security Council but Russia and China are still querying the US-drafted resolution.

Ban Ki-moon, who will be the new UN secretary-general, called for a "clear and strong" resolution to be adopted.

Mr Ban was formally elected by the UN General Assembly on Friday to replace Kofi Annan at the end of the year.

Throughout the week there has been uncertainty about whether North Korea carried out a nuclear test, tried to but failed, or merely made a false claim, the BBC's Nick Miles reports from Washington.

The apparent confirmation that Pyongyang did carry out a nuclear test could make the passing of the UN resolution more likely, our correspondent says.

Fizzle theory

US scientists found that there were traces of radioactive gas in the air near the site of last Monday's alleged nuclear test.

White House officials cautioned that this result alone did not confirm a successful test but it could mean that a nuclear test had been attempted.

One official quoted by the Associated Press said the results could indicate a "nuclear fizzle", rather than a full test.

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Both South Korean and Chinese scientists said earlier they had detected no evidence of radioactivity in air, soil and rainwater tests.

The US has revised a draft resolution to remove the threat of imminent military action in an effort to allay Chinese and Russian concerns.

The US envoy to the UN, John Bolton, said he expected the vote to be held on Saturday but it depended on Russia and China's stance.

But Russian Foreign Minister Sergei Lavrov said the current draft still had "elements that should be discussed and clarified".

The US draft retains a controversial provision allowing nations to inspect cargo moving in and out of North Korea in pursuit of non-conventional weapons.

Clouds gather

It urges Pyongyang to implement a September 2005 agreement in which it pledged to give up its nuclear programme in exchange for aid and security guarantees.

A Russian envoy who has just visited North Korea said he urged the Pyongyang authorities to return to six-nation disarmament talks.

Alexander Alexeyev said his meetings were "useful" but did not say how the North Koreans had responded to his appeal, the Associated Press reported.

New Japanese sanctions against North Korea have already come into force.

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The measures include trade and travel bans, barring North Korean ships from Japan's ports, and freezing imports and visits by North Korean officials.

South Korean President Roh Moo-hyun has held talks in Beijing with his Chinese counterpart, Hu Jintao.

A top South Korean official said the two men did not discuss the details of the draft resolution but agreed in general principle to support UN action.

Mr Roh is facing increasing pressure at home to reverse his so-called "sunshine policy" of engagement with the North.

As many as 3,000 protesters gathered outside Seoul city hall to demand the government cut off aid and investment.