High seas may force the US navy to postpone an attempt to shoot down a failed spy satellite, the defence department said this afternoon.

The USS Lake Erie, a cruiser that has the job of firing missiles at the satellite, is caught in bad weather in the north Pacific.

"We don't anticipate the weather being good enough today," an officer told the Associated Press.

However, the source said conditions could improve sufficiently in the next few hours for an effort to be made. A final decision will be made by the defence secretary, Robert Gates.

The Pentagon had been waiting for the space shuttle Atlantis to return to Earth before launching the missiles.

"We're now into the window," a senior military officer said after the shuttle landed today.

The US military wants to shoot down the satellite as a "precautionary measure", but critics say the move is a step towards the militarisation of space.

The Bush administration says it is necessary to destroy the satellite - which failed shortly after its 2006 launch - because it is carrying toxic rocket fuel. Nasa fears the craft could pose a risk to populated areas as it falls back to earth.

Critics say the use of missiles to destroy the satellite is further evidence of the Pentagon's desire to test its space weapons.

The US navy has modified its Aegis anti-missile radar system for the exercise, making clear that its missile defences can quickly be adapted to shoot down a satellite.

Targeting the satellite will allow the Pentagon to test key elements of its space defence system, including identification and tracking, at the joint space operations centre at Vandenberg air force base in California.

"Whatever their motivation for shooting down the satellite, it's clear that this will be quite useful to the military," Joan Johnson-Freese, an expert on military space issues at the naval war college in Newport, Rhode Island, told the Washington Post.

The US defence department has been developing weapons that can shoot down satellites or missiles for decades. In 1985, the air force successfully tested an air-launched missile to shoot down a satellite.

Early last year, China earned widespread condemnation when it shot a ground-based missile at an old satellite in space, amid fears of a space arms race that would include the targeting of satellites.

Thomas Fingar, a deputy director of national intelligence, told Congress last week it would not be difficult to inflict serious damage on US defence capabilities by targeting its satellites.

China and Russia have pushed for a UN treaty to ban weapons in space. The US, which abrogated the antiballistic missile treaty in 2002, has opposed the proposal.