WASHINGTON (Reuters) - The Pentagon said on Thursday it was very confident that a spectacular and unprecedented missile strike from a U.S. Navy warship had destroyed the toxic fuel tank of a defunct U.S. spy satellite.

The 1,000 pound (450 kg) fuel tank could have released its load of hydrazine fuel as a toxic gas if it had fallen to Earth, causing health risks for anyone nearby, officials said.

The strike took place 153 miles above the Pacific Ocean on Wednesday as the satellite sped through space at more than 17,000 mph (27,000 kph), the Pentagon reported.

“This was uncharted territory. The technical degree of difficulty was significant here,” said Marine Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff.

“You can imagine, at the point of intercept, there were a few cheers that went up in operations centers and on that ship,” Cartwright told reporters at the Pentagon.

Both Russia and China had expressed concern ahead of the mission.

Moscow said it could be used as cover to test a new space weapon. China, criticized by Washington after it shot down one of its own satellites last year, said the operation may influence security in space and harm other countries.

U.S. Defense Secretary Robert Gates said Washington could share some information to reassure China about the operation.

“Our whole approach to this was one of complete transparency, prior notifications and letting everybody know what was going on and the purpose of the activity,” Gates told reporters during a visit to Hawaii.

Washington said the only reason for the mission was to prevent harm to humans from 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,” Cartwright said, putting the chances that the tank had been breached at around 90 percent.

FIREBALL SUGGESTS SUCCESS

Cartwright said a fireball in video images of the strike, a vapor 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.

The USS Lake Erie launches a Standard Missile-3 at a non-functioning National Reconnaissance Office satellite as it traveled in space at more than 17,000 mph over the Pacific Ocean, February 20, 2008. REUTERS/U.S. Navy/Handout

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 5,000-pound (2,300 kg) satellite was struck by an SM-3 missile fired from the USS Lake Erie northwest of Hawaii at 10:26 p.m. EST, the Pentagon said.

The operation was the first time a sea-based missile has been used to hit a satellite, according to experts. The United States and the Soviet Union conducted anti-satellite tests in the Cold War but used other techniques.

The operation used modified elements of the U.S. missile defense system and supporters of that project were quick to praise the operation. But Gates said it should not be seen as a test of the missile defense system’s viability.

“We’ve had a number of successful tests,” he said. “I think the issue of whether it will work is behind us. We just need to keep improving its capabilities.”

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.

U.S. 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 China’s anti-satellite test.

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.

The Pentagon has said the stray spacecraft was a test satellite for the National Reconnaissance Office, a U.S. intelligence agency, that was launched in December 2006 and stopped communicating within a few hours of reaching orbit.