General James Cartwright told reporters it would be 24 to 48 hours before a full confirmation would be available on the fuel tank filled with 450kg of toxic hydrazine.

The Defence Department has a 'high degree of confidence we hit the tank' but 'we can't say for sure' at this time, he said at a Pentagon press conference held less than nine hours after the strike.

A senior Pentagon official earlier had said the missile appeared to have struck the fuel tank containing hydrazine, which could have leaked toxic gas over a wide area if it had survived re-entry into the Earth's atmosphere.

'The intent here was to preserve human life...it was the hydrazine we were after,' Gen Cartwright, who is vice-chairman of the Joint Chiefs of Staff, said rejecting Moscow's suggestion it was an anti-missile test.

'This is a modified system, this isn't a missile defence system,' he said.

The operation had raised concerns that the United States was trying to test an anti-satellite weapon, amid rising global tensions about the militarisation of space.

Beijing responded swiftly, calling on Washington to provide more information and warning of potential international consequences.

'China is continuing to closely follow the possible harm caused by the US action to outer space security and relevant countries,' said Foreign Ministry spokesman Liu Jianchao.

China caused an international outcry when it shot down one of its own weather satellites on Jan 11 last year in what was widely seen as an anti-satellite test.

US Defence Secretary Robert Gates gave the go-ahead for the missile strike as he flew from Washington to Honolulu, a base for the three Aegis warships involved in the intercept attempt.

The USS Lake Erie, a guided missile cruiser, fired a single modified tactical SM-3 missile that hit the schoolbus-size satellite travelling at more 11,200kmh, the Pentagon said.

While Pentagon officials have said satellite debris will re-enter the Earth's atmosphere immediately due to the relatively low altitude at which the satellite was intercepted, Gen Cartwright said the US has not seen debris touch the Earth's surface.

Most of the debris is expected to burn up on re-entry within two days. But it could take up to 40 days for all the debris to re-enter.

Russia's Defence Ministry said on Sunday that the US plans looked like a veiled weapons test and an 'attempt to move the arms race into space'.

US officials have insisted the aim was to prevent potential risk to humans from the de-orbiting satellite, and not to test an anti-satellite weapon or keep its secrets from falling into the wrong hands.

Admiral Timothy Keating, head of the US Pacific Command, acknowledged similarities with the Chinese shootdown but said this was significantly different as the US gave public notice first.

'They just shot, they didn't tell anybody about it,' he said.

Adm Keating said he has been in contact with his counterparts in the region and their response had been, he said, 'thanks for the call, good luck, keep us posted'.

The Lake Erie was carrying two missiles in case the first attempt failed. The USS Decatur, an Aegis destroyer, was armed with a third missile, while the Aegis destroyer USS Russell tracked the event at Pearl Harbor.

The warships are equipped with radars capable of tracking a medium range missile warhead in space, and armed with SM-3 interceptor missiles.

Software changes were made to the missile used so that it could recognise a satellite as its target rather than a ballistic missile.

AGENCE FRANCE-PRESSE, REUTERS, LOS ANGELES TIMES