The Pentagon (search) is extending tours of duty in Iraq for about 1,500 soldiers, mainly helicopter and other transportation support personnel, defense officials said Monday.

It is the latest in a series of adjustments to a plan for replacing the approximately 125,000 troops who have served in Iraq nearly a year with a slightly smaller, more mobile force.

In this case, commanders determined that allowing the 1,500 soldiers to leave as originally scheduled would create an unacceptable gap in capability. So they will remain in place -- some for as few as five extra days, others for as much as 60 more days, officials said.

The decision is expected to be publicly announced as early as Tuesday, officials said.

In preparing for the force rotation in Iraq, the Pentagon pledged to make every effort to avoid requiring any troops to remain more than 12 months, but some if not all of the 1,500 whose tours are being extended will exceed the 12-month target, the officials said, speaking on condition of anonymity.

The 1,500 are a mix of Army active-duty and National Guard and Reserve soldiers. Officials who discussed some aspects of the decision did not reveal the specific units involved.

This is not the first change to the Pentagon's rotation plan.

In mid-December Defense Secretary Donald H. Rumsfeld (search) approved a plan to send the 1st Brigade of the 82nd Airborne Division to Iraq for up to four months to fill a gap left by a National Guard infantry brigade that needed extra time to train for its Iraq mission. Also, the 82nd's 3rd Brigade was told it will remain in Iraq for two months longer than originally planned.

In November, the Pentagon said three additional battalions of Marines would be part of the rotational force.

The rotation has gotten under way in recent days with the departure of some U.S-based troops and ships laden with equipment, although the bulk of the movements to and from Iraq will happen in February, March and April. Some members of the 101st Airborne Division have returned home.

The Army also is rotating its Afghanistan force. The 10th Mountain Division, which had been scheduled to leave in February after completing six months in Afghanistan will remain an extra three months. Its designated replacement, the 25th Infantry Division, will do a 12-month tour instead of six months.