“I do think it is important that this be conditions-based,” Admiral Michael Mullen, the chairman of the Joint Chiefs of Staff, told reporters.
A US-Iraqi agreement approved over the weekend by the Iraqi cabinet calls for all 150,000 US troops to be out of the country by the end of 2001 regardless of the conditions on the ground.
President-Elect Barack Obama set an even tighter deadline of 16 months during the campaign.
In a television interview on Sunday, Mr Obama said he would call in the Joint Chiefs after his inauguration and “start executing a plan that draws down our troops”.
Adml Mullen said he would offer his advice to the new president, who takes office on January 20, and then follow his orders.
“Should president-elect Obama give me direction, I would carry that out. I mean, that’s what I do as a senior member of the military.”
Referring to the 2011 deadline contained in the so-called Status of Forces Agreement reached with Baghdad, Mullen said, “I certainly understand the boundaries.”
But he suggested the deal might be revisited at some point between now and then.
“And so three years is a long time. Conditions could change in that period of time,” said Adml Mullen, adding the United States will continue to talk with Baghdad “as conditions continue to evolve.”
Asked if the agreement could be changed, he said “that’s theoretically possible”.
Adml Mullen said he had discussed the agreement with General David Petraeus, the commander of US forces in Middle East and southwest Asia, and General Raymond Odierno, the US commander in Iraq.
“We’re all very comfortable that we have what we need. Conditions continue to improve,” he said.
“Clearly, moving forward in a measured way, tied to conditions as they continue to evolve over time is important,” he added.
Adml Mullen said it would take two to three years to safely withdraw all US forces from Iraq.
“It is very doable, but it’s not the kind of thing that we could do overnight,” he said.
“To remove the entire force would be, you know, two to three years, as opposed to something we could do in a very short period of time, as we’ve looked at it thus far.
“Clearly, we’d want to be able to do it safely. So when I talk about that kind of range of time, it really is conditioned by what’s going on,” he said.