The Marine Corps is planning to call up as many as 2,500 Marine reservists for combat duty in Iraq and Afghanistan, dipping into a rarely used pool of troops to fill growing personnel gaps in units scheduled to deploy in coming months, officials said yesterday.

It is the first time the Marines have resorted to involuntary call-ups since the initial invasion of Iraq in March 2003, when about 2,000 Marines were ordered into service for a short duration. It means thousands of Marines across the country who have left active service could soon be forced to return.

They would come from a pool of about 59,000 members of the Individual Ready Reserve (IRR) -- Marines with specific skills who left active duty and returned to civilian lives, but are obligated to serve if called. Marine Corps officials said yesterday that reservists in their first or last years of enrollment will not be subject to recall.

Officials said they will try to choose Marines with the smallest number of combat tours, leaving about 35,000 subject to the call-up.

Those receiving a recall notice will have five months to report to active duty and could serve tours of 12 to 18 months, Marine officials said yesterday.

President Bush authorized the involuntary recall on July 26, and officials are waiting to determine what skills are in short supply for deployments early next year before sending out notices to individual Marines. The authorization allows the Corps a maximum of 2,500 involuntary recalls at any one time.

The authority for involuntary recalls is until the end of the "Global War on Terror" -- a conflict for which there is no realistic end in sight. That could mean thousands of Marines could be involuntary activated over coming years.

Col. Guy A. Stratton, who is in charge of the Marines' manpower mobilization plans, told reporters at the Pentagon yesterday that the service is seeing a "shortfall" of volunteers from the IRR who previously offered to go back to active duty to fill gaps in units deploying overseas.

Stratton said that the "volunteer numbers are on a downward trend," and that outreach to certain specialists in recent months has met with resistance.

The shortfall, Stratton said, means that the Marines need about 1,200 people in a number of areas, from infantrymen to engineers, military police, intelligence personnel, aviation mechanics and truck drivers -- even though 1,366 members of the IRR were still on active duty as of Monday.

By creating a pool of 2,500, the Marines could train and deploy enough troops to fill the gap for two rotations.

"Since this is going to be a long war, we want to be able to use all our assets," Stratton said.

The Army has used its IRR several times since the Sept. 11, 2001, attacks. It has mobilized about 5,000 soldiers from that pool over the past five years, most of them since the middle of 2004, the Army said.

Some members of Congress have characterized the involuntary recalls and other measures as a "back-door draft" employed because wars have overly stressed the military. Other measures include "stop-loss" orders, which keep soldiers on active duty longer than expected.

Jack Reed (D-R.I.), a member of the Senate Armed Services Committee, said in a statement that the involuntary recall reflects the "wear and tear" on the U.S. military.

"The drain on our soldiers, their families and the military's resources caused by today's operations in Iraq and Afghanistan need to be addressed immediately, or there will be severe long term consequences for the nation and our military," Reed said.

Retired Army Maj. Donald E. Vandergriff, who has studied military personnel issues, said the IRR call-ups are both a necessary and an expected part of the military's wartime operations.

"The IRR is there to fill the needs when there are gaps," Vandergriff said. "From the viewpoint of the services, they're using a tool to help them accomplish a mission that has been assigned. That's what that tool is there for."

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