A series of coordinated guerrilla offensives over four months has raised serious questions about the ability of the U.S.-backed Salvadoran armed forces to respond effectively to an increasingly potent rebel military threat.

The recent guerrilla successes have led Salvadoran officers and their U.S. advisers to the grudging conclusion that the Army, as it now stands, does not have the manpower, training or planning capability to defeat the insurgents.

From the Pentagon to lonely mountain garrisons in Morazan province, the key to reversing this situation is seen as sharply increased U.S. aid. But there is also concern here that the Salvadorans have not used the money and instructions already given them to best advantage.

Since it began three years ago, the Salvadoran civil war has passed through several such periods when guerrilla fortunes seemed to be on the rise, only to fall again later. But the latest rebel advances--longer lasting and broader than in the past--seem to mark a watershed, presenting Washington with a choice between investing more money and diplomatic capital or facing the prospect of a continued insurgency at a level not previously envisaged.

One route to this crossroads passed through Berlin, a once prosperous coffee town overlooking the fertile Lempa River valley. With a population swollen to 30,000 by refugees from villages to the south, Berlin was the biggest city that the rebels had ever taken. They moved in Jan. 30 and held on for four days before pulling away.

The rebels were able to occupy Berlin that long, despite rocket attacks by Air Force planes, because almost the entire effective fighting force of the Salvadoran Army was tied down in Morazan province, more than 50 rugged miles to the northeast, in the nearest thing this war has seen to a conventional, set-piece battle.

The Salvadoran government has a 22,000-man Army, with 11,000 more in the National Guard and police forces. But it has come to rely almost exclusively on three U.S.-trained battalions and two special "minibattalions" to handle heavy combat.

"As shoes are made for walking, we're made for counterinsurgency," explained a captain in the Ramon Belloso Battalion, trained last year at Ft. Bragg, N.C.

These units together total fewer than 5,000 men against a guerrilla fighting force also estimated by Salvadoran officers to number about 5,000. The other soldiers in the regular Army and security forces are basically serving, as one U.S. adviser said with frustration, "guarding bridges."

The guerrillas are well aware of this.

"There is a clear difference between the ways the Atlacatl, Belloso and Atonal battalions fight and the run-of-the-mill troops. Their quality is better and they are more aggressive," said Julio Ama, a rebel officer interviewed in the town of La Palma last week.

"But lately they've been so battered that we've been calling them the 'rubber band battalions' because they have to stretch them from Chalatenango to Morazan to Usulutan."

Since launching the current string of offensives Oct. 10, guerrillas of the Farabundo Marti National Liberation Front have drawn the Army into heavy fighting in traditional rebel redoubts in Chalatenango and Morzan provinces. Other units in the five-faction guerrilla front branched out from hideouts in Usulutan during the past two weeks to capture Berlin and attack two bridges on the province's main highway, severing east-west traffic for the crucial cotton industry of which Usulutan is a center.

The traditional role of much of the Salvadoran military has been to man small garrisons in towns all over the country. There is still pressure for them to do this from influential landowners interested in protecting their property and from an increasingly nervous and vulnerable population.

But senior Salvadoran commanders have come to recognize that this is impossible, given the current strength of the guerrillas.

"It's illogical to think that we are going to have a sufficient force in every town to defend it from the subversives," the commander of the Atlacatl Battalion said in an interview. "On the contrary, we need to go where they are."

But as the Army tries to make this transition, the effect has been to abandon some towns to the rebels, sometimes without a fight, while in many cases still not organizing forces to seek them out on their own ground.

Col. Mario Enrique Acevedo, commander in the provincial capital of Morazan, explained why he withdrew the 24-man garrison of civilian defense and security forces from the village of Sociedad last week.

"If I leave the 24, they'll kill five, or two, and the rest will be captured. Then they release them, but they're useless. They capture 24 G3 rifles and 12,000 cartridges. This way," said the colonel, "they don't get anything. Not even a pistol."

But when asked why neither his troops nor the Belloso Battalion, now working out of Perquin to the north, move in on a major guerrilla camp about six miles above Sociedad, Acevedo said it is difficult to carry out such operations without more U.S. help.

Such conflicting needs and pressures have helped provoke open dissent in the Salvadoran officer corps. After a six-day mutiny against him last month, Defense Minister Jose Guillermo Garcia's personal authority was badly crippled. As a result, other commanders are jockeying for position in a reorganization of the general staff and Defense Ministry that has led to predictions that Garcia himself is on the way out.

"This is a war run by a committee, not a commander," said one political analyst who works closely with senior government officials.

U.S. diplomats do not call increased aid a panacea, but they do insist that without it things will get worse. At a press conference Friday, U.S. Ambassador Deane R. Hinton said congressional refusal to increase military expenditures here as requested by the Reagan administration, "has given new heart to the guerrillas."

The administration got only $26 million of the $60 million in military aid it requested for El Salvador this year. It is looking to almost quadruple that amount for fiscal 1984, to $86.3 million. Of that, by far the largest increase is in outright military grants, which would rise from $8.5 million this year to $55 million next year if Congress approves.

Foreign Minister Fidel Chavez Mena said this week he would also like to get a congressional change of heart for this year, restoring military aid to "at least the $60 million requested by the administration."

While the talk of aid goes on in El Salvador and Washington, the psychological effect of guerrilla occupation in towns such as this one has become considerable.

"People are beginning to mistrust the Army's ability to protect them," said a leftist intellectual in the capital.

Here in Berlin, the threat takes on a hard outline.

After the guerrillas left town on Feb. 2, several companies of soldiers and National Guardsmen occupied the city. They included camouflage-clad troops from the Ramon Belloso Battalion called down urgently from the Morazan operation.

But on Friday, the last major National Guard unit pulled out, and the garrison here was once again reduced to fewer than 100 policemen.

Refugees coming from neighboring villages report guerrillas as close as El Tablon, less than three miles away, and rumors are widespread of another possible attack.

"We're still threatened here," said a visibly shaken municipal employe as he watched the National Guard pull out while a handful of policemen in civilian clothes patrolled the street. "They walk around without uniforms so they won't be targets. I've never seen anything like this."