US commanders also are considering holding back the deployment of a third army brigade to Afghanistan after a NATO-led force expands into the southern part of the country next year, a US defence official said.
"The hope is that the conditions will permit some drawdowns of troops," said US Defense Secretary Donald Rumsfeld after meeting with lawmakers.
"As you know we plussed up considerably from 137,000 to, I believe, up to about 160,000 to be helpful during the election period, and we certainly expect to go back down to about 137,000 and if conditions permit we can go just below that," he said.
A Pentagon spokesman said no decisions have been made.
"The planning accounts for the full range of possibilities. It will be the commanders' assessment of the conditions on the ground that will prompt any adjustments of force levels," said Lieutenant Colonel Barry Venable.
Troops delayed
But the defence official who asked not to be identified said the deployment of the 1st Infantry Division's 1st Brigade from Fort Riley, Kansas, which was supposed to be in Iraq by early January, already has been delayed.
The Germany-based 1st Armored Division's 2nd Brigade, which is now in Kuwait preparing to deploy inside Iraq, also has been placed on hold, the official said.
"One's delayed and one's on standby," the official said. Each has about 5,000 troops, he added.
"What General Casey and his commanders have to decide given the situation, given the increased capability of the Iraqi security forces, is there the opportunity to reduce the US presence? That is being evaluated at this time," the official said.
General George Casey is the commander of the 155,000 US troops now in Iraq.
"One option is we could delay or not deploy those two brigades. The other is to use one of the two to provide trainers for the Iraqi force," the official said.
He said the top priority at the moment is to get through the December 15 elections in Iraq and then bring home the forces that are scheduled to come out of the country.
That would bring US forces levels down to about 138,000 troops, the baseline military presence that the United States has maintained in Iraq since April 2004.
A decision to cancel the deployment of the brigades from the 1st Infantry Division or the 1st Infantry Division would mark the first cut below that level.
Decisions on deeper cuts could come within weeks of the elections, the defence official said. "Things could happen very rapidly," he said.
But he stressed that commanders were intent on preserving their flexibility.
"Any reduction will be phased in very slowly to make sure commanders won't lose combat power if it's needed," he said.
General Casey in the past has expressed confidence that "fairly substantial" reductions in US forces in Iraq can be made next year if key political milestones are met and Iraqi forces assume more of the security burden.
President George W Bush, who has been touting the progress made in Iraq, also is under intense domestic political pressure to bring home US troops.
Afghanistan
The commander of US forces in Afghanistan, meanwhile, said US force levels there could come down next year in light of a NATO decision Thursday to expand a NATO-led peacekeeping force into more volatile south.
"If NATO does move down to the south, clearly I can expect with the adjustment of forces there could be less US presence in that region,"
Lieutenant General Karl Eikenberry told reporters.
Lt Gen Eikenberry would not say by how much the 18,000-strong US force might be reduced.
But the US defense official said an option under consideration was to cancel the scheduled deployment of a brigade from the army's 10th Mountain Division.