The Reagan administration has directed the military services to "revitalize and enhance" special forces whose missions would include operating behind the lines in Warsaw Pact countries in a war, informed sources said recently.

The military spearhead of such forces in Europe, as plans stand now, would be Army troopers trained in Warsaw Pact languages and in such sabotage tactics as blowing up railroads and command posts.

The Army gives such training, much of it cloaked in secrecy, at Fort Bragg, N.C., among other places. Army troopers with behind-the-lines specialities also are rotated to bases in West Germany where they stand ready to slip into eastern Europe.

Sources said that if war broke out, specially trained Army troopers operating out of such a forward base as Bad Tolz, between Munich and Innsbruck, would be likely to draw saboteur duty in such Soviet satellite countries as East Germany, Poland, Hungary and Czechoslovakia. In addition, they said, the Central Intelligence Agency would work less visibly with resistance groups they hope will spring up in those countries.

In confirming military preparations to exploit Soviet vulnerabilities behind the NATO front line, Sen. Sam Nunn (D-Ga.), a member of the Armed Services Committee who is familiar with war plans, said recently that "to concede the Soviet Union safe territory in eastern Europe, when they have considerable problems politically and economically, and perhaps militarily, in any kind of war would not be a rational NATO posture."

In Pentagon guidance to the armed services for fiscal years 1984 through 1988, which has not been made public, the Reagan administration calls for upgrading mobile forces for duties beyond that of regular Army units.

"We will maintain highly ready special operations forces as vital adjuncts to conventional military power," the guidance states. "Apart from their utility in conflicts at higher levels of intensity, special operations forces provide the United States options to respond to contingencies of a nature which would render inappropriate the employment of regular military forces."

Becoming more specific in discussing responses to a Soviet push across the NATO line, the new guidance states that "to exploit political, economic and military weaknesses within the Warsaw Pact and to disrupt enemy rear area operations, special operations forces will conduct operations in eastern Europe and in the northern and southern NATO regions. Other opportunities for counteroffensives against Soviet interests, forces and proxy forces worldwide will be exploited to the extent feasible."

The guidance makes clear that the special forces the Reagan administration wants upgraded would combat "low-level aggression," presumably through the Green Beret type of advisory role the Army emphasized in the early days of the Vietnam war, as well as operate behind the lines in Europe and elsewhere during periods of tension or war.

Giving a tall order which would seem to require a bigger Army than the one projected in the Pentagon's five-year plan, the guidance states, "We must revitalize and enhance special operations forces capability to project U.S. power where the use of conventional forces would be premature, inappropriate or infeasible. In a war with the Soviet Union, special operations forces will be capable of conducting the full spectrum of special operations on a worldwide basis."

In setting an objective for 1986, the guidance tells the services to "ensure that special operations forces are sized, structured and specially trained and equipped to be able to exploit Soviet and surrogate vulnerabilities in Europe, Northeast Asia, Southwest Asia and Latin America and contribute to efforts to defeat Soviet and surrogate initiatives."

Veteran Pentagon observers said previous administrations also set high goals for special forces but were constrained by other demands on the Army, which has not been expanded significantly since the Vietnam war.

Addressing the administration's call for special forces trained to operate within the Warsaw Pact during a war, Robert W. Komer, director of defense policy at the Pentagon under President Carter, said "we have had such plans and programs for the last two decades."