The US is confident that its shooting down of a disabled spy satellite with a missile managed to destroy its potentially toxic fuel tank.
Marine Gen James Cartwright said there was a 80-90% chance that the satellite's tank had been destroyed.
A fire ball, vapour cloud and spectral analysis indicating the presence of hydrazine all indicated that the tank had been hit, he told reporters.
The operation has been criticised by China and Russia.
"We're very confident that we hit the satellite," Gen Cartwright said at a Pentagon briefing hours after the missile was fired.
"We also have a high degree of confidence that we got the tank."
It would take another 24-48 hours for officials to confirm whether the operations had been completely successful, he said.
Owner: National Reconnaissance Office
Launched: 14 Dec 2006
Weight: 2,300 kg (5,000lbs)
1,134kg (2,500lbs) could survive re-entry
Carrying hydrazine thruster fuel
Gen Cartwright said he could not rule out that hazardous material might fall to earth, but said there was no evidence of this happening so far.
He added that officials would continue to track debris falling over the Atlantic and Pacific Oceans over the next two days.
"Thus far we've seen nothing larger than a football," he said.
The satellite, USA 193, was struck 153 nautical miles (283 km) above earth by an SM-3 missile fired from a warship in waters west of Hawaii.
Operatives had only a 10-second window to hit the satellite, which went out of control shortly after it was launched in December 2006.
The missile needed to pierce the bus-sized satellite's fuel tank, containing more than 450kg (1,000lbs) of toxic hydrazine, which was otherwise expected to survive re-entry.
USA 193 lost control shortly after launch on a Delta II rocket
China called on the US on Thursday to provide more information about the mission.
Russia suspects the operation was a cover to test anti-satellite technology under the US missile defence programme.
The US denies the operation was a response to an anti-satellite test carried out by China last year, which prompted fears of a space arms race.
US officials had said that without an attempt to destroy the fuel tank, and with the satellite's thermal control system gone, the fuel would have been frozen solid, allowing the tank to resist the heat of re-entry.
If the tank were to have landed intact, it could have leaked toxic gas over a wide area - harming or killing humans if inhaled, officials had warned.
"The intent here was to preserve human life... it was the hydrazine we were after," Gen Cartwright said on Thursday.
The US has also denied that it shot down the satellite to prevent parts of it from falling into the hands of foreign powers.
Gen Cartwright said most of the satellite's intelligence value was likely to have been destroyed.
1 SM-3 missile launched from a US Navy cruiser in Pacific Ocean
2 The three-stage missile headed for collision location, where the relative "closing" speed was expected to be 10km/s (22,000mph)
3 Satellite came in range at altitude of 247km (133 nautical miles), close to edge of Earth's atmosphere
4 Missile made contact with satellite with objective of breaking fuel tank, freeing hydrazine into space
5 Much of the debris will burn up but an as yet unknown amount is expected to be scattered over hundreds of kilometres