The Pentagon insisted no decision has been made on future US troop levels in Iraq as lawmakers voiced alarm over reports the White House is looking at retaining as few as 3,000 troops on the ground.
Iraqi leaders have announced plans to open talks with Washington on keeping a US military force beyond an end-of-year deadline, but the size of possible future mission has yet to be agreed.
The Pentagon faced questions about troop plans after Fox News, CNN and the New York Times reported President Barack Obama's advisers were considering an option that would leave a smaller force of 3,000 troops in Iraq, down from the roughly 46,000 now in place.
Fox News and the Times, citing unnamed officials, also said Defense Secretary Leon Panetta had endorsed the 3,000 troop option.
But Panetta declined to specify what troop options were on the table and said the size of a post-2011 force would depend on the outcome of discussions with Baghdad.
"No decision has been made with regard to numbers of troops," Panetta told reporters during a visit to the new 9/11 memorial in New York City.
"I can't give you a number, or tell you what that number looks like. It's going to have to be part of the negotiations," he said.
Under a 2008 security agreement with Iraq, the United States must withdraw its troops from the country by the end of the year, unless the two sides agree a new deal.
The Pentagon chief has previously suggested Iraqi leaders want the US military to maintain a presence in the country and that talks will focus on hammering out the details.
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He said Tuesday that he was not aware of any troop number proposed by the Iraqi government but "they have indicated a desire for trainers to be there."
According to a transcript of an interview Tuesday on PBS's "Charlie Rose Show," Panetta said that "one thing is clear, certainly it's not going to be a large number of troops that are present ...particularly if it's a training mission."
Both US and Iraqi military officers say Iraq's forces need outside assistance to defend the country's air space, territorial waters and borders.
But the US troop presence remains a delicate political issue Iraq, and anti-US Shiite cleric Moqtada al-Sadra has warned of "war" if American forces stay beyond 2011.
In Washington, Obama faces competing demands over the pace of a US drawdown, with some members of his own party urging a complete withdrawal by the end of the year.
But several lawmakers Tuesday expressed concern that a force of 3,000 could jeopardize stability in Iraq.
"I think it's a mistake," Senate Intelligence Committee Chairwoman Dianne Feinstein told reporters. "I think it's too fast."
Three other senators, who strongly backed the US troop "surge" in 2007, issued a joint statement saying they were deeply troubled about reports of a possible smaller force.
"This is dramatically lower than what our military leaders have consistently told us over the course of repeated visits to Iraq that they require, and that is needed to support Iraq in safeguarding the hard-won gains that our two nations have achieved at such great cost," said the statement from Republican senators John McCain, Lindsey Graham and independent Joe Lieberman.
Speculation about the size of a future American force has often focused on a range of 10,000 to 15,000 troops.
The New York Times wrote that the US commander in Iraq, General Lloyd Austin, has proposed a larger force to stay in the country, about 14,000 to 18,000 troops.