It marks the first time the Army has called on the Individual Ready Reserve, as this category of reservists is known, in substantial numbers since the 1991 Gulf War. Several hundred of them have volunteered for active-duty service since the Sept. 11, 2001 terrorist attacks.

Those who are part of the involuntary call up are likely to be assigned to National Guard or Reserve units that have been mobilized for duty in Iraq or Afghanistan, according to Army officials who discussed some details Tuesday on condition they not be identified because a public announcement was planned for Wednesday.

Members of Congress were being notified of the decision Tuesday, the officials said.

Unlike members of the National Guard and Reserve, the individual reservists do not perform regularly scheduled training. Any former enlisted soldier who did not serve at least eight years on active duty is in the Individual Ready Reserve pool, as are all officers who have not resigned their commission.

The Army has been reviewing its list of 118,000 eligible individual reservists for several weeks in search of qualified people in certain high-priority skill areas like civil affairs.