US Defense Secretary Donald Rumsfeld said the force rotation will depend in part on how quickly security duties can be shifted to Iraqi forces as well as how many other countries contribute troops to the US-led occupation.

"It's important to emphasize that the rotations next year will not be driven by timelines for force reductions but rather by the security situation on the ground in Iraq," Rumsfeld told reporters.

"We're committed to staying as long as necessary with as many forces as necessary to successfully complete the mission," he said.

General Peter Pace, vice chairman of the Joint Chiefs of Staff, said three enhanced combat brigades already have been alerted to prepare for the rotation. Combat support units will be alerted in the next two to three weeks, he said.

"The next rotation will have fewer heavy units, like tanks, and more units that have Humvees and lighter vehicles for mobile infantry," he said.

"And as you do that, not only on the combat side, that also lightens up the mount of logistic support needed and mechanics needed and the like to keep the heavy equipment moving," he said.

He said the Pentagon was going through "a very, very detailed scrub, unit by unit," to decide what combat support units will be deployed and what the balance will be between reserve and active duty troops.

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