Department of Defense spokesman Bryan Whitman said the satellite, which has lost power, would land "over the next several weeks ... late February, early March time-frame."

"We are aware of it, we are monitoring it," he told reporters, declining to give any more details about the satellite or say where it might strike.

The satellite's impending fall from orbit has given rise to worries that it might leak out highly toxic substances.

The New York Times cited satellite monitoring experts who believe it is an experimental imagery satellite launched in 2006.

The United States has a thick web of billion-dollar satellites monitoring the Earth, some including high-powered telescopes or radars, with the capability to zoom in and help launch precision strikes on enemy targets.

"We track all man-made objects that are orbiting the Earth," Whitman said.

"Since we've been in the business of doing that, for 50 years or so, there have been more than 17,000 man-made objects that have re-entered the Earth atmosphere."

Spy satellites are frequently maneuvered in space, in relatively low orbit, to meet military surveillance needs, requiring them to be tanked up with highly toxic hydrazine fuel, according to specialists.

Hydrazine is harmful to the human central nervous system and can be fatal in big doses. However, it breaks down quickly in heat and ultra-violet light, the French security agency Ineris said in a report.

Specialists cited in the New York Times said the hydrazine would burn off if the fuel tank breaks, as is likely, when re-entering the atmosphere.

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