The US military has successfully shot down one of its own satellites that was falling to Earth without direction.

The spy satellite was launched in December 2006 but a systems failure meant that it was unable to establish control links with Earth.

It has since drifted downwards and was due to enter the atmosphere during the next month.

US authorities decided to destroy the satellite to avoid it hitting populated areas, as it was fully loaded with toxic fuel.

A missile was fired from the USS Lake Erie in the Pacific Ocean at 3.26am GMT which hit the satellite.

"The objective was to rupture the fuel tank to dissipate approximately 453kg of hydrazine, a hazardous fuel which could pose a danger to people on Earth, before it entered into Earth's atmosphere," said the US Department of Defense.

"Due to the relatively low altitude of the satellite at the time of the engagement, debris will begin to re-enter the atmosphere immediately.

"Nearly all of the debris will burn up on re-entry within 24 to 48 hours and the remaining debris should re-enter within 40 days."

The fate of the fuel tank and its contents is not known at this time, but it is expected to have ruptured and dispersed its contents into the atmosphere.

China and Russia expressed strong concerns about the missile strike. Chinese authorities said that the decision could spark a new arms race in space. The Chinese government shot down one of its own satellites last year.

Many military and civilian functions are now totally dependent on satellite data, particularly from the US GPS system.

Europe and China are already working on their own versions of GPS.