Just south of the provincial capital of Usulutan, past a final Army outpost and onto a dirt road, the prosperous coffee and cotton plantations give way to low mountains covered with trees and underbrush and dotted with small farms.

On the paved highway from San Salvador, 55 miles to the west, peasants carrying machetes, the traditional tool of their farming trade, are common sights. But here, at the entrance to guerrilla territory, peasants walked along the road with automatic rifles slung over their shoulders.

A few more miles down the road, where a bunker of piled stones commanded the view behind us, we parked and were greeted by two men and a woman carrying U.S.-made M16 rifles.

In the course of an afternoon last week, these guerrillas and others who are part of the war against El Salvador's U.S.-backed government showed us the camp they said was established nine months ago as command headquarters for all forces in the Farabundo Marti Liberation Front in what they call the southeastern zone, stretching from the Lempa River to the Gulf of Fonseca separating El Salvador from Nicaragua.

There are approximately 600 people at the camp and 5,000 peasants living in the surrounding rural area the camp controls, according to one bearded man at the headquarters. The armed peasants, guerrillas said, are members of the militia. They are armed to protect their homes and the camp but do not go out on missions, according to the guerrillas.

The commander of the camp and the southeastern zone, Juan Ramon Medrano, who is a member of the guerrilla coalition's policy-making board, the United Revolutionary Directorate, said that in recent military campaigns government troops had ceased their practice of the wholesale killing of noncombatant peasants in guerrilla-held areas. He said the peasants had not been bothered in this area during an unsuccessful government offensive in the province in October.

Medrano would not give exact figures on the population of the region or the camp. But an hour-long interview with him, as well as an afternoon of touring the camp and witnessing troop training and practice maneuvers, provided a picture of what appeared to be a highly trained guerrilla force that was adequately fed and armed with automatic weapons. Discipline and morale appeared high, and the guerrillas made no effort to conceal the camp buildings from detection from the air.

An Army duty officer back at the Usulutan garrison, where new fortifications were visible around the grounds, said the Army was aware of the strong guerrilla presence in the area but did not plan to move against them again in the near future.

"We have other, more important, things to do," he said. "This is the harvest season, and we have to protect the trucks on the road."

Although accounts of the progress of the country's civil war vary greatly as told the by government or by Radio Venceremos, the clandestine guerrilla station, the existence of the Usulutan camp and its apparently secure position demonstrates the Salvadoran Army's scant success in preventing the guerrillas from holding territory or expanding into new areas of the country.

Despite the insistence by Salvadoran President Jose Napoleon Duarte and Defense Minister Jose Guillermo Garcia that the government is in control of the entire country, there appears to be a military deadlock in this part of Usulutan Province. Both sides are able to make brief incursions into enemy strongholds, but neither is capable of remaining long or destroying the other's military force.

The zone has more than 100 miles of coastline riddled with swampy inlets providing easy access for sea-borne supplies. Unlike the areas of heavy guerrilla concentration in northern El Salvador, the area around Usulutan has some of the country's richest cotton and coffee farm land and is densely populated.

Medrano and a battalion commander who headed guerrilla forces against a government offensive in October acknowledged that their presence here did not mean they had defeated the Army troops, but that the government had abandoned the offensive after several weeks and the guerrillas were able to return to their camp.

Medrano, whose identity was confirmed by Salvadoran and diplomatic sources, said the guerrillas now move and operate throughout the country and have begun to take the initiative in direct actions against government troops. Overall, he said, the FMLN forces have moved away from the tactic of "defensive resistance" they had employed to recoup the setbacks of a general offensive they launched and lost last January.

"Militarily, we feel that we are winning the war, we are defeating the junta." Medrano said.

Although he has denied the war is stalemated, President Duarte has said his armed forces must be built up to 40,000 to 50,000 troops from the present 20,000 to eradicate the guerrilla force he estimated to total 3,000 to 4,000 soldiers.

U.S. government estimates of the guerrilla force are somewhat higher: 4,500 to 6,000 guerrilla irregulars and several thousand peasant militia members. U.S. Ambassador Dean Hinton said in an interview this week that the guerrillas have been able to replace their losses and maintain troop strength at about the same level as at the time of the offensive last January.

"The basic problem," Hinton said, "is that the force superiority of the Army is in the range of four or six to one, depending on your estimate of the guerrillas, and that's just not enough." He said new weaponry, such as recoilless rifles and machine guns and apparently ample supplies of ammunition in guerrilla hands this year are evidence they are receiving outside supplies.

At the Usulutan camp men and women were seen carrying weapons constantly and with a nonchalance that was at times disconcerting. Of the scores of automatic weapons seen, the most common were well worn M16s, the standard weapon of U.S. troops in the Vietnam War. The peasants we had seen at the entrance to the camp, we were told, were militia members who were armed to protect their homes and the guerrilla base but do not normally go on missions outside the camp.

Also in the militia are teen-agers and younger children. Walking up to the main part of the camp on a donkey path, we saw adults holding drills for groups of about 30 young girls and 60 boys, about a fourth of whom had old weapons.

Late in the day, a column of about 100 uniformed guerrillas, including five women, marched rapidly up the steep path, many drenched in sweat from the day's exercises. One carried a light mortar, but most of those in the column did not have guns.

In the main camp there was a one-room house with a dirt floor that served as an office and school. Nearby was a bomb shelter tunneled into the side of a hill. Rolando Julian, a barrel-chested physician, said food and medical supplies for the camp were obtained by peasants in the area.

A squadron of uniformed soldiers marched smartly through the high archway and iron gate in front of the school with much shouting of commands. They stood at attention and presented arms as Medrano and two other guerrilla officers marched into the small area. A grin escaping from one of the soldiers suggested that the martial demonstration was perhaps not the customary way of greeting their commander.

Medrano, in the interview, said he had been a full-time guerrilla since 1973, when he discontinued sociology studies at the National University. He said he belongs to the Revolutionary Army of the People, one of the five guerrilla groups in the FMLN, all of which have proclaimed at least vague adherence to Marxist-Leninist ideology of armed struggle.

But Medrano seemed intent on dispelling the guerrillas' reputation for radicalism. U.S. and Salvadoran government officials have based their refusal to endorse a negotiated end to the Salvadoran war on the charge that the guerrilla forces will impose a Marxist totalitarian system on El Salvador if they win the war and will shunt aside their moderate political allies--two center leftist parties making up the Democratic Revolutionary Front.

"We have been maturing little by little and we have demonstrated sufficient awareness of the complexity of the world and international politics of the moment to know we cannot impose any kind of radical government from one day to the next," he said. "And even more we believe that a radical plan of government would be injurious to our country and simply provoke or justify attitudes of belligerency toward us.

"The war had become unnecessary . . . . We are ready to die to the last soldier to take San Salvador if necessary, but if before that they recognize us justly as a representative of the people . . . we are ready to resolve this" through negotiation.

In a statement that seemed to go beyond the opposition's previous demand for changes in the armed forces as a condition for a political settlement, Medrano excluded the Army from the demand.

"If someone might have said at some time that we want the destruction of the Army, we are sure that that was indeed an error. We consider that there are only a few fascist elements in the Army. The institution as such can remain as long as it adheres to the guidelines of a government of broad national participation."

Medrano said that for a cease-fire and a "political solution there has to be a duality of military power. We are not going to trust any kind of guarantee that does not contemplate the recognition of military power for our organization."

He stopped short of naming Cuba as a source of the guerrillas' arms and supplies, but he said that "if Reagan sends tons of weapons and millions of dollars to the junta, to a totally unpopular regime, we have the right to have all countries of the world give us money and arms and every other kind of assistance."

The guerrilla commander said his group "believes in elections" but that the elections for a constituent assembly to be held in March are a way to "legitimize a new junta" and are a "facade for the decision to proceed with total war against the people."