White House says Iraqi leaders have met only half of political and security goals.
That will bring the number of US, bringing the number of US army ground combat brigades down from 20 to 19.
The Iraq troop reduction is to begin in December and will be completed by July 2008 under a plan recommended by General David Petraeus, the top US commander in Iraq.
Speaking at the Pentagon on Tuesday, Lieutenant-General Carter Ham, the chief of operations for the Joint Chiefs of Staff, said the withdrawal pace “isn’t mechanical” and will depend on the security situation.
“It isn’t that once it’s in motion it will proceed apace … at each step along the way commanders will make an assessment: Can we go faster? Do we need to go slower?”
Meanwhile, violence continued to rage across Iraq with bomb attacks near an Iraqi army checkpoint and a police station in Baghdad on Tuesday.
A car bomb in central Baghdad killed at least three Iraqi soldiers and three civilians.
The blast occurred in the same square where private security guards protecting US diplomats killed Iraqi civilians in September.
In western Baghdad, a car bomb killed at least six people and wounded 25 as families were returning home from holiday outings.
At least six people were killed in another attack when a parked car blew up near a petrol station across the road from the army checkpoint. Dozens more were wounded.
In Mosul, a tanker blast killed five and wounded 80 people when an explosives-laden sewage truck blew up near a police station which has been targeted several times.
Police said the explosion brought down the building that was recently rebuilt, killing at least four policemen including the station chief, and leaving 75 people wounded.