For 15 days in August, 3,000 troops swept through the moutains north of this provincial capital in an operation that military experts here consider a model of what the United States would like the Army of El Salvador to do more often.

"A showpiece based on hard intelligence," one western observer called it.

Now, five months later, as the military impact of that campaign can be evaluated more closely, its considerable cost to the civilian population is becoming obvious in refugee camps and orphanages around the country.

When the fighting ended in August, the government claimed what some military men call a "score" of 256 dead guerrillas to three dead soldiers.

But about 1,000 leftist guerrilla combatants remain in this single central province, half of them in the region where the offensive took place, according to Col. Juan Pablo Galvez, the deputy commander of the 5th Brigade here.

In its August sweep the Army also interrupted the harvests of peasants in the guerrilla-controlled areas and seized quantities of their food, as brigade commander Col. Napoleon Alvarado noted.

That move, in retrospect, appears to have dealt a heavy blow to the guerrillas' "masses" or civilian collaborators if not to the insurgent troops themselves.

Since late December the rebels have been sending noncombatants into government-held areas. These include starving children, their bellies swollen and their arms shriveled, their hair losing its color from advanced malnutrition; desperately ill women whose gaunt, strained faces reflect months of homeless wandering; old men afraid to talk about anything except how afraid they have been.

Workers for the Green Cross, a local organization that runs several refugee centers, say that they have received more than 600 people from San Vicente Province in the last several weeks. Others have been able to find refuge with family or friends. Still others have been taken in by the government.

A group of 23 children, 13 women and one 72-year-old man were picked up by troops shortly after dawn here Tuesday and are now living, somewhat nervously, here in the San Vicente barracks.

In one rural refugee center this afternoon a 24-year-old woman told of fleeing her home in August amid intense fighting. "So many people died," she said. "Old people. Women about to have children."

She and more than 200 others, she said, have not been back to their little settlements around the village of San Ildefonso since.

"We passed these months living beneath the trees," said another woman from that party, which reached a Green Cross refugee center two days after Christmas. With her husband's whereabouts unknown, she and her four children survived eating "small leaves and roots," she said.

At another camp earlier this week a grizzled old man named Atanasio from near the town of Santa Clara said he and a group of about 60 others spent as long as 22 days--a figure he repeated with a precision unusual among peasants--without real food. "Many people died," said Atanasio. "Some from hunger, some from the cold. There were storms that swept through."

Petrona Alvarado (no relation to the colonel) is one of the women here in the barracks who was picked up Tuesday. She said her group had been helped to a hill above the main highway by nine armed guerrillas who then let them make their own way down to the road at about 5 a.m. where they accidentally ran into a military patrol.

She and the others said the guerrillas had ordered them to leave earlier in the week.

The insurgent troops of the Central American Revolutionary Workers' Party guerrilla faction in the area around San Ildefonso "have no use for us anymore," said the lone old man in the group here. Only the young people stayed behind, he said, as soldiers from the barracks stood nearby listening.

The old man said all boys from 14 to 18 and all girls from 16 to 20 remained behind in what he called the "military schools" of the insurgents at the settlement of La Presa near the town of Amatitan.

The man said a woman revolutionary workers' party commander called "Rita" had given the order to move out all civilians last month.

The August operation, commanded by Col. Alvarado and Lt. Col. Sigifredo Ochoa who recently led a six-day mutiny in nearby Cabanas Province, used a variety of tactics that are innovative for this war.

Frequently this Army's best U.S.-trained battalions are sent on sweeps and "blocking maneuvers" for a period of about a week, much as is happening in Morazan province to the northeast.

In the August "Operation Azeno Palma," however, Col. Alvarado said the troops packed up and left on a Friday but came back in again on Saturday in a surprise move and fought for another week.

Another tactic repeatedly recommended by the United States and for which the crack battalions are trained is a night ambush using small patrols moving constantly in the field.

Cols. Alvarado and Galvez, despite claims to the contrary by some U.S. advisers, say they continue to use this strategy and get their best results that way. Alvarado said one such ambush is planned for tonight.

But as a retired Salvadoran captain put it, alluding to the famous insurgent maxim that the guerrilla is a fish whose sea is the civilian population, "El Salvador's guerrillas move their sea around with them."

Night ambushes, some officers here concede, take a high civilian toll in this war.

Col. Alvarado has said one reason he has not used the tactic more is concern for noncombatants.

Petrona, who seemed intent on speaking out about what she had been through, said that in August she and her family were forced out of their home by fighting that killed "more than 200" people. "There were men, children, women," she said, "all mixed together, dead."

Petrona said that she felt reasonably safe now as she stood talking and nursing her three-month-old child.

"But last night, no," she said. "Because all the boys here [soldiers] were going around saying they were going to kill us. But joking. Kidding. But it made us nervous."

Col. Galvez, smiling and speaking in front of the group, explained to two reporters that "these people tell you people one thing and us another."

Galvez added subsequently that 19 prisoners were taken in Operation Azone Palma and have since been returned to their families except for one who was wounded and turned over to a priest. That priest said this afternoon that no prisoners have ever been turned over to him. "That's not what they usually do to prisoners," he said.