Thousands of soldiers who had expected to retire or otherwise leave the military will be required to stay if their units are ordered to Iraq or Afghanistan.

The announcement Wednesday, an expansion of a program called "stop-loss," affects units that are 90 days or less from deploying, said Lt. Gen. Frank L. "Buster" Hagenbeck, the Army's deputy chief of staff for personnel.

Commanders can make exceptions for soldiers with special circumstances. Otherwise, soldiers won't be able to leave the service or transfer from their units until they return to their bases after their deployments end.

The Army is struggling to find fresh units to continue the occupation of Iraq. Almost every combat unit has faced or will face duty there or in Afghanistan, and increased violence has forced the deployment of an additional 20,000 troops to the Iraq region, straining units even further.

The move allows the Army to keep units together as they deploy, Hagenbeck said. Units with new recruits or recently transferred soldiers would not perform as well because the troops would not have worked together.

Since the Sept. 11, 2001, attacks, every Army unit ordered to Iraq, Kuwait, Afghanistan and nearby countries has faced a similar rule, although it has been applied in a piecemeal fashion. Army officials portrayed Wednesday's announcement as an administrative change.

Initially, the expanded order will affect several units about to go to Iraq: most of the 2nd Brigade of the 10th Mountain Division, from Fort Drum, N.Y.; the 265th Infantry Brigade of the Louisiana National Guard; the 116th Armored Brigade of the Idaho National Guard; the 278th Armored Cavalry Regiment of the Tennessee National Guard, and the 42nd Infantry Division's headquarters staff, from the New York National Guard.

The 2nd Brigade of the 2nd Infantry Division, a South Korea-based unit, is expected to deploy later this summer and will be subject to the expanded stop-loss program as well, officials said.

There has been criticism of the program as contrary to the concept of an all-volunteer military force. Soldiers planning to retire face more months away from their families and homes.

In an opinion piece in Wednesday's New York Times, Andrew Exum, a former Army captain who served under Hagenbeck in the 10th Mountain Division in Afghanistan, called the treatment "shameful."

"Many, if not most, of the soldiers in this latest Iraq-bound wave are already veterans of several tours in Iraq and Afghanistan," he wrote. "They have honorably completed their active-duty obligations. But like draftees, they have been conscripted to meet the additional needs in Iraq."