A missile launched from a Navy ship struck a dying US spy satellite passing 130 miles (210 kilometers) over the Pacific, the Pentagon said.
It was not clear whether the operation succeeded in its main goal of destroying a tank aboard the satellite that carried toxic fuel that US officials said could pose a hazard to humans if it landed in a populated area.
"Confirmation that the fuel tank has been fragmented should be available within 24 hours," the Pentagon said in a written statement late on Wednesday.
The USS Lake Erie, armed with an SM-3 missile designed to knock down incoming missiles -- not orbiting satellites -- launched the attack at 10:26 pm EST (0326 GMT Thursday), according to the Pentagon. It hit the satellite as the spacecraft traveled at more than 17,000 mph (27,000 kilometers per hour).
Because the satellite was orbiting at a relatively low altitude at the time it was hit by the missile, debris will begin to re-enter the Earth's atmosphere immediately, the Pentagon statement said. "Nearly all of the debris will burn up on re-entry within 24-48 hours and the remaining debris should re-enter within 40 days," it said.
The use of the Navy missile amounted to an unprecedented use of components of the Pentagon's missile defense system, designed to shoot down hostile ballistic missiles in flight -- not kill satellites.
The operation was so extraordinary, with such intense international publicity and political ramifications, that Defense Secretary Robert Gates, rather than a military commander, was to make the final decision to pull the trigger.
The government organised hazardous materials teams, under the code name "Burnt Frost," to be flown to the site of any dangerous or otherwise sensitive debris that might land in the United States or elsewhere.
Also, six federal response groups that are positioned across the country by the Federal Emergency Management Agency have been alerted but not activated, FEMA spokesman James McIntyre said. "These are purely precautionary and preparedness actions only," he said.