About 132,000 US soldiers are currently serving in Iraq

More than 40,000 National Guard and reserve forces will also be called up as part of a rotation of forces, US Defence Secretary Donald Rumsfeld said.

The rotation means an overall reduction of forces now in Iraq, currently numbering around 132,000.

But US Joint Chief of Staff Richard Myers said the reduction in numbers did not mean a reduction in capabilities.

General Myers also said that security was not the only US priority in Iraq, as issues surrounding the country's economy, future government and infrastructure remained.

The announcement came on the day President George W Bush signed an emergency spending package for Iraq and Afghanistan worth $87.5bn.

The number of US troops in Iraq was expected to fall as more Iraqis were trained to take over their role, despite calls from some senior figures in US Congress for numbers to be boosted.

BBC Pentagon correspondent Nick Childs says that, with the US army stretched thin, planners have been juggling troop numbers for weeks to fashion a new rotation plan.

Insurgents shot and killed a Polish army officer south of Baghdad on Thursday - the first soldier to die from a multi-national division set up to ease the pressure on US forces.