The Reagan administration, seeking ways to reverse a deteriorating military situation in El Salvador, is considering allowing U.S. military advisers to operate closer to Salvadoran government troops in the field.

Officials stress that such a move, long advocated by U.S. military specialists, would not involve any combat role or going on operations with government forces against leftist guerrillas.

But it would allow some U.S. advisers to operate at a brigade headquarters or provincial command center in the countryside, where they might have greater influence over troops they have helped train and, perhaps more important, over the Salvadoran field commanders.

American officials say they believe that poor leadership by some field commanders, especially those who are political appointees, is a major factor in the generally lackluster performance of the Salvadoran army.

Thus far, the U.S. advisers, 37 of whom are now in El Salvador, have been restricted to the capital city of San Salvador, except for occasional quick journeys into the field.

American officials say guerrilla forces now are moving into and out of more towns with little punishment from government forces and are capturing more government troops and weapons than ever before in the three-year-old war.

The tactics of the estimated 5,000 to 6,000 guerrillas are described by these officials as increasingly "audacious," while the ability of the U.S.-backed 22,000-man Salvadoran army to cope with the rebels increasingly is being questioned here.

That gloomy military assessment is behind the administration's fresh public emphasis on the situation in the tiny Central American country, its request in Congress for another $60 million in military aid and the thought it is giving to increasing the number of U.S. military trainers there.

Interviews with officials in the White House, Pentagon and State Department produce a unanimous view that the military situation has deteriorated significantly in the past six months.

Yet, these officials say, the essential ingredient in the effort to keep El Salvador from falling to leftist forces is to prevent a defeatist attitude from developing. The strategy of the governments here and in San Salvador is based upon having enough military strength to prevent a rebel victory. The theory is that this ability to maintain the political center eventually will lead leftist leaders to cooperate and keep the right wing from splitting off.

Officials say there is no "concrete evidence" on which to claim that the war is being lost. The army is not disintegrating. The rebels are not holding ground that they temporarily capture. And the population shows no sign of commitment to the rebel side.

Nevertheless, there is a uniform view here that things are now going badly, and it is not clear that more money alone will solve the problem, which many specialists believe lies in the attitude of Salvadoran military and political leaders toward prosecuting the battle.

Specialists here say that many field commanders are unwilling to use unconventional and aggressive tactics in offense or in counterattacking the rebels. Contrary to the advice of American trainers, the Salvadorans continue to rely on massive and expensive daytime sweeps through the countryside with battalion-sized units rather than smaller scale operations.

Coordination between units is said to be poor, with a key element of an operation apt to be missing when the time comes to attack. The roughly 4,500 soldiers and junior officers trained by Americans are said, by Americans, to be performing reasonably well in the field. But these are said to be about the only real combat forces available and are therefore spread too thinly to counter the like-sized guerrilla forces.

A particular dilemma surrounds Gen. Jose Guillermo Garcia, the Salvadoran defense minister. While a strong supporter of agricultural and other democratic reforms backed by Washington, Garcia also has appointed many political allies to key army posts. This protects Garcia from his foes and keeps the army generally in support of the reforms, but it has weakened combat leadership seriously, U.S. officials say.