WASHINGTON (AP) _ A Pentagon plan to use perhaps 2,000 or 3,000 reservists in any U.S. force sent to Bosnia for peacekeeping is an example of increasing reliance on America's citizen soldiers.

Reservists, who typically are in uniform one weekend a month and for an annual two-week training session, make up about one-third of the total U.S. military. But they play a more active role now than during the Cold War.

Gen. John Shalikashvili, chairman of the Joint Chiefs of Staff, told senators Tuesday of the likelihood that reservists would be given a Bosnia peacekeeping operation as part of a U.S. contingent of 20,000, heavily armed troops.

He gave no details on what missions they might perform. Typically, however, an Army-dominated operation such as Bosnia would require reservists for combat support jobs such as heavy-lift helicopter crews, medics and transportation crews.

It was not immediately clear whether the Pentagon intended to amass the 2,000 to 3,000 reservists by asking for individual volunteers, as is often done, or by requesting presidential authority to mobilize entire units.

In providing refueling, fighter and transport aircraft for such high-paced operations as the U.N. enforcement of ``no-fly'' zones over Bosnia and parts of Iraq and to provide aid in Rwanda in recent years, the Air National Guard managed to fill all its crew requirements with volunteers.

Although they get less public attention than the active-duty soldiers, airmen, sailors and Marines, members of the National Guard and Reserve components have been involved in every substantial U.S. military operation since the end of the Cold War _ whether combat, humanitarian aid abroad or disaster relief at home.

``With the total force getting smaller, they are coming to rely on the Guard and Reserve more and more. That's a fact of life,'' said Maj. Robert Dunlap, a spokesman for the National Guard Bureau at the Pentagon.

The largest-scale call-up of reservists in recent years was during the months leading up to the 1991 Persian Gulf War. Some 225,000 members of the National Guard and Reserves were called to active duty, and about half were sent to the war zone.

Some examples of the reservists' importance to the active-duty force:

_The Army has 100 percent of its water supply battalions, heavy helicopter units and judge advocate general units in the Army National Guard and Army Reserve. Most of its medical brigades, civil affairs units and oil supply battalions also are in the reserves.

_A growing number of heavy bombers in the Air Force are now in reserve units. These include B-52s in the Air Force Reserve and B-1Bs in the Air National Guard.

_The Naval Reserve has five new classes of ships assigned to it. These include coastal mine-hunting, tank-landing and mine-countermeasure vessels. Also, the aircraft carrier USS John F. Kennedy is being used to train reserve aviators.

Reservists have been involved in a wide variety of overseas missions in recent years, including:

_Haiti. Three Army National Guard military police companies with about 400 soldiers were mobilized to replace active-duty Army MPs during the U.N. intervention there. About 165 Guard special forces soldiers from Colorado, California, Alabama and Massachusetts also were used in Haiti in January 1995.

_Somalia. In February 1994, an Army National Guard team assisted in the hand-off of the peacekeeping mission by training Pakistani pilots and others.

Reservists also have contributed to disaster-relief operations in the United States. More than 5,000 National Guard members helped respond to the Flint River flood in Georgia in 1994, for example, and reservists lent a hand for tornado relief in the Carolinas, Virginia and Wisconsin last year.

Some counternarcotics operations also are performed by Army National Guard members.