Guerrilla forces of El Salvador's Farabundo Marti National Liberation Front carried out what appears to have been a major attack on a garrison in Arcatao, a small town about 25 miles south of this provincial capital, in an indication of increased guerrilla action and tactical coordination.

The guerrillas' radio station, which operates in the northeastern area of the country, hailed yesterday's attack in today's broadcast, indicating that the action was planned and coordinated by rebel headquarters, possibly to draw the Army's resources away from a large offensive it is conducting at San Vicente volcano in the central part of the country.

Approximately 1,500 government troops, garrisoned in the provincial capital of San Vicente, have been trying to drive off guerrillas who have been based on the volcano for almost a year. Several Army checkpoints in the area were reported under guerrilla attack while the Arcatao assault was going on.

Radio communications between the Arcatao garrison and Army headquarters in San Salvador, monitored by an amateur radio operator in the capital yesterday, told a dramatic story. At midmorning on Wednesday, the garrison radio operator notified his superiors that a group of guerrillas was attacking the town, and called for reinforcements.

As thee day progressed, the operator's pleas became more frantic. He described a column of 600 uniformed guerrillas equipped with machine guns progressively encircling the garrison. Heavy shooting could be heard in the background.

Late in the afternoon the operator said the column was made up of 1,000 guerrillas. Ammunition was parachuted in, but the operator said the guerrillas got to the drop first. The transmission ended in the late afternoon with the words, "It's finished."

It is unlikely that the guerrillas would use up to a thousand men to attack a garrison where only a few platoons are stationed, as reported by the Army radio operator. But even if the guerrillas numbered only a fraction of that figure, the operation indicates that the Farabundo Marti forces are operating at a level of military efficiency never seen during their unsuccessful offensive in January.

Approximately 200 troops are permanently stationed in Arcatao, which lies just across the border from Honduras.

A military spokesman at the Chalatenango Army barracks today denied that a major enemy assault took place. "It was just a little ambush," the officer said. "Some of our soldiers aren't used to real fighting and panic easily. That's why the radio operator sounded so worried."

However, he confirmed four soliders dead, and a National Guard officer here said the Air Force was being asked to send in aerial reinforcements to provide "support bombing."

United Press International quoted an official in Arcatao who said the rebels blitzed a house defended by troops with bazooka fire killing 14 soldiers and wounding at least as many.

[Army commanders radioed that helicopters were unable to drop troop reinforcements because of intense rebel fire and the town was inaccessible by ground because key bridges in the area were destroyed by guerrilla demolition teams.]

The rebel radio said at least 140 soliders were killed, but the guerrillas often exaggerate the number of casualties they have inflicted.

The Chalatenango area, and Arcatao in particular, has long been a stronghold of leftist peasant organizations. Since January, it has been what is called a military zone, where the guerrillas roam more or less unmolested in the countryside while government troops make periodic patrols from their fortified garrisons in the towns.

Villages like Arcatao were deserted by the civilian population months ago.

Fighting is also starting again in the northeastern province of Morazan, where the guerrillas are said to have hospitals, farming lands and their clandestine radio base. The Army conducted a pacification campaign in the province last month.

Officers interviewed during a recent visit to San Vicente said they had destroyed 12 guerrilla camps and killed more than 200 rebels. Officers in the combat zone said privately, however, that this figure was exaggerated.

The reality of the war here vanishes behind similar confusions and contradictions on every front. One officer said there are about 1,000 armed guerrillas on the volcano, but the booty of the 12 camps reportedly destroyed so far consists of enough antibiotics to treat about a dozen wounded guerrillas as well as five rusty hunting rifles.

One officer said there were no prisoners because guerrillas prefer to commit suicide rather than fall captive and face interrogation. Another said there are prisoners but they are dealt with by a special department.

Similarly, with the question of the civilian population, one officer said there is none on the volcano. The Red Cross said it accepted 119 women and children from the San Vicente area, delivered by the Army.

The Army's elite Atlacatl Brigade, which has received U.S. training, heads the San Vicente operation. Its members look sharper and more disciplined than the lethargic troops in other barracks.

The courtyard of the garrison is decorated with a mural depicting the enormous face of a blond woman with tearful blue eyes next to a life-sized commando blasting his way through the shrubbery in typical action comics style. The real-life soldiers, small and dark and sparing in their movements, are a sharp contrast.

Much of the action on the volcano involves softening up the guerrilla lines with artillery day and night. In the past, the Army has relied heavily on aerial bombardment to flush out the guerrillas and their supporters, but it is now the rainy season and, to the frustration of the Air Force, the magnificent volcano remains shrouded in clouds.