A large United States spy satellite weighing about 20,000lb has lost power and could slam into Earth in late February or early March, government officials said Saturday.

The satellite, which no longer can be controlled, could contain hazardous materials, and it is unknown where on the planet it might come down, they said. The officials spoke on condition of anonymity because the information is classified as secret. It was not clear how long ago the satellite lost power, or under what circumstances.

Gordon Johndroe, a spokesman for the National Security Council, said, "Numerous satellites over the years have come out of orbit and fallen harmlessly. We are looking at potential options to mitigate any possible damage this satellite may cause.''

The spacecraft contains hydrazine, which is rocket fuel, a government official divulged. Hydrazine is a toxic chemical and can cause harm to anyone who comes into contact with it.

An uncontrolled re-entry could risk exposure of US secrets, said John Pike, a defence and intelligence expert. Spy satellites typically are disposed of through a controlled re-entry into the ocean so that no one else can access the spacecraft, he said.

Pike also said it was not likely the threat from the satellite could be eliminated by shooting it down with a missile because that would create debris that would then re-enter the atmosphere and burn up or hit the ground.

The satellite is estimated to be the size of a small bus.

In 1979, a 78-ton abandoned space station fell from orbit into the Indian Ocean.

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