A US warship is moving into position in the western Pacific in preparation to shoot down an out-of-control spy satellite before it re-enters Earth's atmosphere, Pentagon officials have said.

The first attempt to destroy the secret US satellite - said to be about the size of a bus - could take place as early as Wednesday night US time.

US military officials say the satellite is carrying highly toxic hydrazine rocket fuel that could be dangerous if it fell on a populated area.

The decision to shoot it down has been criticised by China which has hinted that it believes the move is a cover for testing anti-satellite weaponry.

A Chinese foreign ministry spokesman told reporters: "Relevant departments of China are closely watching the situation and working out preventive measures".

Last year, China was itself criticised by the US and several of its allies which accused Beijing of risking a space arms race after it used a ballistic missile to destroy one of its own obsolete weather satellites.

Russia's defence ministry has also said it fears the US plan is a veiled test of US anti-satellite capabilities and represents an "attempt to move the arms race into space".

The ministry said: "The decision to destroy the American satellite does not look harmless as they try to claim, especially at a time when the US has been evading negotiations on the limitation of an arms race in outer space."

Critics have also said the justification of health fears may be a cover for preventing highly-classified spy satellite technology from falling into foreign hands.

The first attempt to shoot down the crippled satellite is expected to take place between 0230 and 0500 GMT on Thursday, US officials have said, with warnings issued for shipping and air traffic to clear the area.

The attempt will involve a modified missile fired from a US warship in waters off Hawaii.

The missile carries a non-explosive "kinetic kill vehicle" – designed essentially to destroy the satellite by smashing into it.

The technique is similar to the system employed in US anti-missile shields.

Speaking in Washington on Tuesday, a Pentagon spokesman said the first attempt would follow the scheduled landing of the space shuttle Atlantis at the Kennedy space centre in Florida on Wednesday at 1406 GMT.

"The window of opportunity … opens as soon as the shuttle is safely on the ground," Geoff Morrell told reporters.

"At that point, we begin to look at when is the best time to take a shot to bring down this dying satellite."

The decision to shoot down the missile was authorised last week by George Bush, the US president, who said the move was based on protecting human health.

The satellite is carrying about half a tonne of hydrazine, a toxic propellant that would have been used to reposition the satellite while in orbit.

US defence officials have said that without intervention, the satellite would crash to Earth sometime in March with little indication where it would land until a few hours beforehand.

PHOTO CAPTION

A Standard Missile-3 (SM-3) launching from the Aegis cruiser USS Lake Erie.