Planners are considering dropping the number to about 100,000 next summer if conditions allow, the officials said on Wednesday.
But a variety of planning scenarios, including the possibility of no cut in troop levels, are being reviewed based on political and security conditions in Iraq and progress in developing US-trained Iraqi security forces, they said.
This comes amid intensifying debate in the US Congress over whether US troops should be withdrawn after 2 1/2 years of war in Iraq.
"The United States military looks at the full range of things that could occur in Iraq and makes plans accordingly, and makes plans for conditions that would lead to a smaller coalition force as well as conditions that would lead to a larger coalition force," said Pentagon spokesman Bryan Whitman.
Whitman said the plan was to drop back to 138,000 troops, considered the recent baseline level for the US force, after the 15 December elections in which Iraqis will select a permanent government.
The Pentagon increased US troop levels in Iraq in advance of the 15 October referendum in which Iraqis approved a constitution, and the US force peaked in October at about 161,000.
After temporarily dropping by several thousand troops, the size of the US force again is rising to help provide security for the elections.
2108 troops have died since the
Iraq invasion, the Pentagon said
In March and again in July, Army General George Casey, the top US commander in Iraq, predicted a "fairly substantial" reduction in American forces next spring and summer if Iraq's political process goes positively and progress is made in developing Iraqi security forces.
Pentagon officials said in August that meant a reduction of perhaps 20,000 to 30,000 troops from the level of 138,000 then in Iraq.
A US defence official, who asked not to be named, said such a cut remains under consideration, but options for a smaller cut or no reduction remain on the table.
"There is the potential over the course of next summer to get to 100,000. Nothing is going to happen fast. It will all be phased," said the official.
"If you start going down below that, you might be sending a message that we're cutting and running," the official added.
The No 2 US commander in Iraq, Army Lieutenant-General John Vines, said on Tuesday that a "precipitous pullout" of US forces would be destabilising to Iraq.
Joseph Lieberman: US will not
leave before its mission is over
The considerations come amid debate in Congress over the future of US involvement in Iraq. Legislator John Murtha of Pennsylvania, an influential Democrat on military affairs who fought as a marine in Vietnam and voted for the Iraq war, called last week for US forces to be withdrawn in six months.
Secretary of State Condoleezza Rice said on Tuesday she suspected American forces "are not going to be needed in the numbers that they're there for all that much longer" due to progress being made by the Iraqis.
Defence officials said the political debate will not drive decisions on troop levels.
There have been 2108 US military deaths in a war that began in March 2003, the Pentagon said on Wednesday, with another 15,804 troops wounded in action. Thousands of Iraqis have been killed.
Senator Joseph Lieberman said on Wednesday in Baghdad that he was encouraged by the political progress in Iraq.
Lieberman, who arrived Wednesday to spend Thanksgiving with US troops, told Iraq's prime minister that American forces will remain in Iraq until their mission is complete, despite growing unease in Congress about the conflict.
"We cannot let extremists and terrorists, a small number, here in Iraq deprive the 27 million Iraqis of what they want which is a better freer life, safer life for themselves and their children", Lieberman said after his meeting Wednesday with Prime Minister Ibrahim al-Jaafari.
The Connecticut Democrat, a member of the Senate Armed Services Committee, said the cost of success in Iraq would be high "but the cost for America of failure in Iraq would be catastrophic - for America, for the Iraqi people and I believe for the world."