"I hate to get into hypotheticals, but I certainly can see circumstances in which we would, first of all, reposition our forces to take them -- to try to move them out of harm's way as much as possible and then see where we go from there," he said.

The defense secretary testified about an administration request for 716.5 billion dollars in defense spending, including more than 235 billion dollars for US military operations in Iraq, Afghanistan and elsewhere this year and in 2008.

Gates reiterated that a precipitous US withdrawal from Iraq would have serious consequences for Iraq and the region and defended the administration's plan to surge more than 21,500 US troops to Iraq to try to end a spiral of sectarian violence.

The United States currently has 148,000 troops in the country.

If the strategy succeeds in tamping down the violence and if the Iraqis move forward on political reconciliation, he said, "I would hope that we would be able to begin drawing down our troops later this year."

Gates said the Pentagon is drawing up a detailed evaluation of the Iraqis' progress in meeting its military and political commitments to the plan, and that Congress would get regular updates.

"We clearly are hoping it will succeed, planning for it to succeed, allocating the resources for it to succeed," he said.

"But I would tell you that I think I would be irresponsible if I weren't thinking about what the alternatives might be if that didn't happen," he said.

The US intelligence community warned in a stark assessment last week of the danger of institutional collapse and greater sectarian upheaval unless measurable progress is made in bringing the violence under control.

Even if the violence is reined in, the Iraqi government will be "hard pressed" to achieve sustained political reconciliation with 18 months to two years, the assessment concluded.

Gates said the plan to stabilize Baghdad with a surge in US and Iraq troops was probably going to slip a few days and become a "rolling implementation," he said.

The secretary acknowledged concern that Iraqi units were arriving in Baghdad at only 55 to 60 percent strength.

"When I was asked this at a press conference last Friday, I said no, that 55 percent wasn't good enough," he told the senators.

Pressed by Senator Carl Levin, the committee chairman, Gates said the United States could interrupt the flow of US troops if the Iraqis failed to live up to their military commitments to the plan.

"I've testified several times that I believe the performance of the Iraqis is absolutely critical to the success of this operation," Gates said.

The Senate Armed Services Committee, meanwhile, voted to approve the nominations of Admiral William Fallon as head of the US Central Command and General George Casey, the departing US commander in Iraq, as army chief of staff.

Fallon, who succeeds General John Abizaid, was approved by a unanimous vote, while the committee voted 14 to three to confirm Casey's nomination.

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