The Pentagon said on Thursday it was very confident that a Navy missile hit the toxic fuel tank of a defunct US spy satellite, which could have caused harm if it had fallen to Earth intact.
The unprecedented strike took place over the Pacific Ocean on Wednesday after a US Navy cruiser launched a missile as the satellite sped through space at more than 27,400 kph (17,000 mph), officials said.
Both Russia and China had expressed concern ahead of the mission, with Moscow suggesting it could be used as cover to test a new space weapon.
But Washington said the only reason for the mission was to prevent harm to humans from the tank of hazardous hydrazine fuel on the bus-sized satellite, which was expected to re-enter the Earth's atmosphere within the next couple of weeks.
"We're very confident that we hit the satellite. We also have a high degree of confidence that we got the tank," said Marine General James Cartwright, vice chairman of the Joint Chiefs of Staff, putting the chances that the tank had been breached at around 90%.
He said a fireball in video images of the strike, a vapour cloud that formed and indications of hydrazine in the air all suggested the tank had been shattered. But he said it could take another 24 to 48 hours to know for sure.
"From our position, you always want to hedge your bet because there's no absolute certainty," Cartwright said.
Debris from the satellite had already started to re-enter the atmosphere over the Atlantic and Pacific oceans but nothing larger than a football had been detected so far, he said.
The 2270 kg (5000 lb) satellite was struck by an SM-3 missile fired from the USS Lake Erie northwest of Hawaii at 2226 EST (0326 GMT Thursday), the Pentagon said.
Wednesday's operation was the first time a sea-based missile has been used to hit a satellite, according to experts. The US and the Soviet Union conducted anti-satellite tests in the Cold War but used other techniques.
The operation used modified elements of its missile defence system. But officials have sought to avoid presenting this mission as a test for that system, saying hitting a satellite is quite different from trying to shoot down a missile.
Some space experts have questioned the Pentagon's justification for the mission, saying the chances of any part of the satellite causing harm were extremely remote.
But Pentagon officials have denied suggestions they wanted to destroy the satellite to prevent part of the classified spacecraft from falling into the hands of rival powers.
"That, unto itself, was not enough reason to go after this satellite with a missile," Cartwright said. "It's the hydrazine that we're focused on."
US officials also have rejected accusations from some security and space experts that the Pentagon was using the operation to test and demonstrate its ability to hit targets in space following an anti-satellite test by China in 2007.
Washington says its case is different from the Chinese test because it was announced in advance and undertaken to protect people. It also says the Chinese craft was struck at a higher altitude, more crowded with other satellites that could be damaged by debris from the destroyed satellite.
A Chinese state newspaper on Thursday accused Washington of hypocrisy for criticising other countries' space ambitions while rejecting a treaty proposed by China and Russia to ban weapons in space and firing a missile at the spy satellite.
The Pentagon has said the stray spacecraft was a test satellite for the National Reconnaissance Office, a US intelligence agency. The satellite launched in December 2006 but stopped communicating within a few hours of reaching orbit.