A large U.S. spy satellite has lost power and could hit the 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.

"Appropriate government agencies are monitoring the situation," said Gordon Johndroe, a spokesman for the National Security Council, when asked about the situation after it was disclosed by other officials. "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."

He would not comment on whether it is possible for the satellite to be shot down by a missile. He said it would be inappropriate to discuss any specifics at this time.

A senior government official said that lawmakers and other nations are being kept apprised of the situation.

The spacecraft contains hydrazine — which is rocket fuel — according to a government official who was not authorized to speak publicly but spoke on condition of anonymity. Hydrazine, a colourless liquid with an ammonia-like odour, is a toxic chemical and can cause harm to anyone who contacts it.

Such an uncontrolled re-entry could risk exposure of U.S. 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's 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.

Satellite the size of small bus

Pike, director of the defence research group GlobalSecurity.org, estimated that the spacecraft weighs about 9,000 kilograms and is the size of a small bus. He said the satellite would create a tenth as much debris as the Columbia space shuttle crash in 2003. Satellites have natural decay periods, and it's possible this one died as long as a year ago and is just now getting ready to re-enter the atmosphere, he said.

Jeffrey Richelson, a senior fellow with the National Security Archive, said the spacecraft likely is a photo reconnaissance satellite. Such eyes in the sky are used to gather visual information from space about adversarial governments and terror groups, including construction at suspected nuclear sites or militant training camps. The satellites also can be used to survey damage from hurricanes, fires and other natural disasters.

The largest uncontrolled re-entry by a NASA spacecraft was Skylab, the 71-tonne abandoned space station that fell from orbit in 1979. Its debris dropped harmlessly into the Indian Ocean and across a remote section of western Australia.

In 2000, NASA engineers successfully directed a safe de-orbit of the 15.4-tonne Compton Gamma Ray Observatory, using rockets aboard the satellite to bring it down in a remote part of the Pacific Ocean.

In 2002, officials believe debris from a 3,175-kilogram science satellite smacked into the Earth's atmosphere and rained down over the Persian Gulf, a few thousand kilometres from where they first predicted it would plummet.