The U.S. Army plans to recall to active duty as many as 5,600 veterans who recently left the service to help fill gaps in Iraq and Afghanistan, officials said yesterday, another signal that the armed forces are stretched thin and that the Pentagon is reaching deep into its reserves to meet its global obligations.

The mandatory call-ups will pull former troops from across the nation back into service, exercising an option in each soldier's enlistment contract that allows the military to insist on their help for several years after their active duty has ended. The last time the Army called back large numbers of soldiers in that category -- the Individual Ready Reserve -- was in 1991 when 20,227 were activated for Operation Desert Shield and Desert Storm in Iraq.

The step -- which Army officers said will be officially announced today -- is one of several the Pentagon has taken recently to augment forces in Iraq as the U.S. commitment there has strained the Army. The Army recently instituted a "stop loss" policy to prevent thousands of deployed troops from leaving the service as scheduled, decided to redeploy more than 3,500 troops in South Korea to Iraq and has extended tours of duty to deal with the insurgency in Iraq.

Instead of reducing U.S. forces in the year since major combat ended, as commanders had once expected, the military has increased its forces to 141,000 as violence has flared. The Pentagon plans to keep at least that many troops through the end of next year to provide security to the fledgling Iraqi government, and officials told Congress last week that the United States probably will have a military presence there for several years.

Some members of Congress said the call-up shows that the Army is too small to handle multiple conflicts and that the increasing burden on the National Guard and Army Reserve could spell long-term trouble for the military.

Sen. Jack Reed (D-R.I.), a member of the Armed Services Committee, said the decision to draw from the Individual Ready Reserve shows that the Bush administration's planning for the war on terrorism was "woefully inadequate." Sen. Susan Collins (R-Maine), who also serves on the panel, said she hopes the call-up will be "a short-term step" while the Army looks for other ways to bolster strength in Iraq through repositioning of troops and restructuring.

Army officials said yesterday that soldiers with specific skills -- mostly in combat support areas such as mechanics, logistics and civil affairs -- will be reinserted into units headed for Iraq over the next six months. Because the veterans are spread nationwide and are not attached to specific Army Reserve units, they will be notified 30 days before their reactivation and will receive a limited amount of additional training. The several thousand soldiers to be called back are among about 111,000 who are eligible as part of the Individual Ready Reserve, officials said.

The advantage to taking such soldiers over recruiting new ones is that the veterans are experienced, skilled and require little retraining. Army officials, speaking on the condition of anonymity because the official announcement had not been made, said the first troops will be notified soon.

"We're a country at war, and we need these soldiers," one Pentagon official said. "We have this pool who are trained, are experienced, know the structure and easily can be put back into the active Army units. This is a very demanding period of time, and we do have this pool of soldiers."

Michael O'Hanlon, a military expert at the Brookings Institution, said using such reserves appears to make sense given the situation in Iraq, but he said it also highlights the "stubbornness" of the Pentagon in resisting a permanent expansion of the Army. Top Pentagon officials have instead argued that the military needs to be streamlined in the post-Cold War era.

Lt. Gen. Theodore G. Stroup Jr., who retired in 1996 as the Army's chief of personnel, activated such soldiers during the Persian Gulf War and said yesterday that it serves a critical purpose in times of conflict. Today's Army, he said, is probably too small and is not configured to deal with the administrative and logistic demands of post-conflict Iraq.

"There will be unhappy people, and there will be people who will be surprised," said Stroup, who added that all soldiers know they may be called back when they enlist. "But it's a national pool of trained manpower that has recent experience."

Sen. John F. Kerry's presidential campaign jumped on the issue yesterday, calling it an "unusual step" that resembles a draft. The campaign released a statement from retired Gen. Wesley K. Clark, who called it a symptom of a "failed do-it-on-the-cheap strategy for Iraq."

"Those to be recalled are the men and women who have already served and were simply trying to get on with their lives," said Clark, a former Democratic presidential candidate.