The US Defence Secretary, Robert Gates, has hinted at bigger cuts in troop numbers in Iraq than those already announced by President Bush.
Mr Gates comments came as a White House report suggested Iraq's government had made little progress in meeting key military and political benchmarks set by the US.
He suggested the current level of more than 160,000 soldiers could be cut to about 100,000 by the end of next year.
On Thursday, President George W Bush ordered gradual troop reductions in Iraq through the first half of 2008.
Under a plan from the US commander in Iraq, General David Petraeus, Mr Bush had said that about 30,000 troops might return by next summer.
Gates however said he hoped Petraeus would be able to recommend an additional drawdown of about five combat brigades, around 20,000 more troops, in the second half of next year.
Speaking at a Pentagon news conference, Gates cautioned that any drawdown would depend on conditions in Iraq.
Asked if withdrawing another five combat brigades in the second half of next year would leave roughly 100,000 troops in Iraq in January 2009, Gates replied: ‘That would be the math.’
A combat brigade ranges from 3,000 to 5,000 troops and the US currently has 20 brigades in Iraq. Withdrawal of combat brigades normally would be accompanied by a reduction in various support forces.
Gates cast the troop reduction plan as the start of a transition in the US mission in Iraq. He called on Democrats, who have sought a faster withdrawal, to support Petraeus' plan and warned against proposals from some lawmakers that would reduce the number of troops available for deployment to Iraq.