Officials speaking on condition of anonymity said the satellite had lost power and propulsion, and could contain hazardous materials.
The satellite contains the rocket fuel hydrazine, a government official told AP on condition of anonymity.
A colourless liquid with an ammonia-like odour, the fuel is a toxic chemical and can cause harm to anyone who comes in contact with it.
John Pike, director of the defense research group GlobalSecurity.org, said an uncontrolled re-entry could risk exposure of US secrets.
Spy satellites typically are disposed of through a controlled re-entry into the ocean so that no one else can access the spacecraft, he was quoted by AP as saying.