"The Department of Defense is currently monitoring the situation," Pentagon spokeswoman Lieutenant Colonel Karen Finn told AFP, confirming that the satellite was "de-orbiting."

She declined to comment on reports, citing government officials, that the satellite contained hazardous substances that may leak after it plunges to the Earth out of control, which according to media reports could happen by late February or early March.

Finn also declined to say what kind of satellite it is. 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.

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

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 Earth's atmosphere.

Some satellites run on nuclear batteries powered by plutonium or enriched uranium, the type used by space probes that have to blast into orbit far above Earth.

"Numerous satellites over the years have come out of orbit and fallen harmlessly," National Security Council spokesman Gordon Johndroe told AFP in an e-mail Saturday.

"We are looking at potential options to mitigate any possible damage this may cause."

In January 1978 a Russian Cosmos 954 nuclear-powered spy satellite crashed into the northern Canadian wilderness.

Another Cosmos satellite broke up over the Indian Ocean in 1983. Traces of its plutonium were detected in snow falling as far away as the southern US state of Arkansas.

Engineers from the US space agency NASA used rockets aboard one satellite to bring it down over the Pacific Ocean in 2000, and officials believe debris from a disintegrated science satellite fell into the Persian Gulf in 2002, the Los Angeles Times reported.

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