Need to know: expert opinion on the satellite operation

America's decision to destroy a defunct spy satellite orbiting Earth by firing a missile into outer space provoked a diplomatic row today, with both China and Russia accusing the US of having carried out a covert weapons test.

The Pentagon claimed that it had fired the missile from the Pacific Ocean to destroy the satellite - which was 133 miles above the Earth - purely because of potentially toxic hydrazine fuel on board the spacecraft which could harm humans.

However, Moscow and Beijing complained that the missile strike smacked of hypocrisy as the US had rejected a joint attempt by the two countries from banning weapons in outer space only a month ago.

And the European space firm Astrium suggested that the Pentagon might also have wanted to destroy the satellite to prevent information aboard falling into the hands of its rival powers, which American officials have denied.

In a briefing today, Marine General James Cartwright, vice chairman of the US Joint Chiefs of Staff, told reporters that the SM-3 missile had been launched from the USS Lake Erie in the Pacific at 10.26 pm local time (0326 GMT), and had struck the satellite at 22,000 miles per hour.

He said that it could take another 24-48 hours to know for sure that the tank containing the hydrazine had been destroyed - but added that the chances of any debris, or hazardous gasses, breaching the earth's atmosphere and harming humans below were remote.

"The intercept occurred. We are very confident that we hit the satellite," he said. "We also have a high degree of confidence that we got the tank."

He added that images of a fireball when the missile struck the satellite, which were shown to journalists, indicated that the hydrazine tank may have been struck. "We have a fireball, and given that there is no fuel, that would indicate that that is a hydrazine fire," he said.

Of the possibility of debris falling, he said that the Pentagon was monitoring the situation, but added: "We have seen nothing yet in the way of ... re-entry, which has survived to hit the earth."

Confirmation that the Pentagon destroyed the spacecraft this morning triggered a fresh diplomatic row with Russia and China.

A Chinese state newspaper, the People's Daily, criticised Washington for hypocrisy for rejecting a treaty to ban weapons in space proposed by Russia and China and then firing a missile at the spy satellite. Washington claims it had rejected the proposed treaty as unworkable, and said it instead favoured confidence-building efforts.

"The United States will not easily abandon its military advantage based on space technology, and it is striving to expand and fully exploit this advantage," the newspaper commentary said.

Speaking at a press conference this morning, Liu Bianca, a Chinese foreign ministry spokesman, said, tersely: "The Chinese side is continuing to closely follow the US action which may influence the security of outer space and may harm other countries."

His words were believed to have been carefully modulated to echo criticisms levelled at Beijing by the Bush Administration when China fired its own ground-based missile into an obsolete weather satellite in January 2007.

US defence officials say their case is different because Washington, unlike Beijing, informed the public and world leaders before firing their missile. They also have insisted the only concern driving the US decision to shoot down the satellite was that the 1,000-pound fuel tank could survive largely intact and release toxic gas.

The Pentagon also denied suggestions they wanted to destroy the satellite to prevent part of the classified spacecraft from falling into the hands of rival powers.

Jeremy Close, a spokesman for Astrium, Europe's biggest defence firm, which employs 3,000 people in the UK, said: "If part of the satellite was to fall intact to the ground, or partially intact, it might yield valuable information about the type of cryptography used by the satellite to communicate with the ground.

"It would be a political coup for an unfriendly country to the US to get hold of that technology from falling debris."

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