The most senior US commander in Iraq has said that progress in bringing security to the country had been uneven and in some cases disappointing.
Mr Bush is under renewed pressure to reduce troop numbers in Iraq
In a letter to troops, General David Petraeus said Iraq's political leaders had not made the gains hoped for under the US troop "surge" strategy.
But he said US forces had achieved "tactical momentum" against insurgents in several areas of Iraq.
Gen Petraeus is to deliver his war assessment to Congress on Monday.
In the letter posted on his Baghdad command's website, Gen Petraeus said this summer was supposed to be a time of "tangible political progress" but "it has not worked out as we hoped".
He thanked the troops for their contributions to the counter-insurgency fight.
"I will go before Congress conscious of the strain on our forces, the sacrifices that you and your families are making, the gains we have made in Iraq," he wrote.
A record 168,000 US troops are now in Iraq after 30,000 arrived in the surge, which started in February 2007 and was completed in mid-June.
Earlier on Thursday US media reported that Gen Petraeus was considering cutting soldier numbers in Iraq by about 4,000.
He may withdraw one brigade if it does not threaten "recent gains" made by US forces since the troop surge, US government and military officials told the New York Times.
He "is worried about risk, and all things being equal he'd like to keep as much as he could for as long as he could", a senior military officer told the newspaper.
The administration's clear hope is that the promise of some reductions will be enough to ease pressure for deeper cuts, correspondents say.
Democrats in Congress have long been pushing for a reduction in troop numbers in Iraq.
On Thursday they seized on a report from a key US security commission recommending a cut in troop numbers by early 2008.
Mr Bush, who decides on troop numbers in Iraq, has said he will follow advice from commanders on the ground, rather than bowing to political pressure, to make his decision.
Correspondents say next week will be critical, with the debate building on whether US troops should stay or leave.