But Bryan Whitman, a Pentagon spokesman, stressed that the defence department was not planning for a quick or wholesale withdrawal of forces.
"Planning for reducing our forces, drawing down our forces, is certainly something that is an appropriate thing to do," Whitman said.
"We are doing that kind of planning, planning for the eventual drawdown, eventual reduction, the beginning of withdrawing of forces from Iraq."
George Bush, the US president, ordered about 30,000 additional troops to Iraq in recent months, bringing the total to about 157,000.
Democrats in congress have been demanding firm plans for a pullout and the American public is growing increasingly impatient with a war that seems to be going nowhere as US casualty figures have passed 3,600.
Gates said he would try to keep senators informed about the "conceptual thinking, factors, considerations, questions and objectives associated with drawdown planning".