Defense Secretary Donald H. Rumsfeld indicated that the newly announced rotation did not necessarily reflect plans for how many American troops will actually be needed in Iraq over the coming two years.

Officials also disclosed that U.S. commanders in Iraq decided they would not need until early next year one brigade, normally numbering about 3,500 soldiers, that was scheduled to deploy to Iraq before the Dec. 15 election. That unit, the 1st Brigade, 1st Infantry Division, based at Fort Riley, Kan., will deploy after the election instead.

The troop rotation announcement identified only six combat brigades, including one from the National Guard, that will deploy over a two-year period beginning in mid-2006. Currently there are about 17 brigades in Iraq. Today's announcement did not include any Marine Corps units, although they apparently will be added later.

The forces, "as presently envisioned," number 92,000 soldiers, the Pentagon said. That compares with the current total of about 160,000, which has expanded this fall in preparation for the Dec. 15 elections. The usual number during most of 2005 was about 138,000, but it's not clear whether that will hold in 2006.

Rumsfeld, appearing before reporters with British Defense Minister John Reid before the troop rotation details were announced, stressed that conditions on the ground in the months ahead will determine any changes in U.S. force levels.

"We're aware of the interest in the press in the mid-to-longer-term levels of U.S. forces and coalition forces in Iraq, but I would caution that it would be a mistake to draw conclusions about such matters when reviewing the force rotation announcements that will be made later today," Rumsfeld said.

"We continue to transition and transfer additional responsibilities to the Iraqi security forces, and the people of Iraq continue to meet the political milestones that they have established," he added. "As these and other conditions are met, Gen. (George) Casey will continue to assess the capabilities that he believes he will need and make recommendations as to the levels he believes will be needed in the period over the coming months."

The Pentagon announced the following major units will deploy as part of the 2006-2008 rotation:

— 1st Brigade, 34th Infantry Division, Minnesota Army National Guard.

— 2nd Brigade, 1st Infantry Division, Schweinfert, Germany.

— 3rd Brigade, 2nd Infantry Division, Fort Lewis, Wash.

— 3rd Brigade, 82nd Airborne Division, Fort Bragg, N.C.

— 3rd Brigade, 25th Infantry Division, Schofield Barracks, Hawaii.

— 2nd Brigade, 10th Mountain Division, Fort Drum, N.Y.

— 13th Corps Support Command, Fort Hood, Texas.

— Division headquarters, 25th Infantry Division, Schofield Barracks, Hawaii.

The announcement said, without elaboration, that decisions made by Rumsfeld "may result in changes to this rotation and may affect units now being identified and advised to prepare to deploy."