A year after its aging Air Force was wrecked by guerrilla saboteurs, El Salvador's use of new warplanes from the United States has brought an added dimension to the fight against leftist rebels.

Having the aircraft on call--chiefly A37 Dragonfly fighters and UH1H Huey helicopters--has given Salvadoran soldiers a military edge and psychological boost in ground encounters, according to Salvadoran and foreign military experts. At the same time, their use in this densely populated country has intensified the danger of civilian casualties and damage to property, handing rebel propagandists a tool in their effort to alienate Salvadorans from the U.S.-backed government.

The double-edged effect was illustrated with devastating clarity earlier this month in the guerrilla takeover of Berlin, a prim market town of 30,000 in the foothills of Tecapa Volcano in Usulutan province. By the time rebel forces withdrew after a four-day occupation, a dozen buildings on the edge of a central square lay flattened and burned. Townsfolk said the damage was done by exchanges between guerrillas and National Guardsmen and by Air Force planes that they said made repeated attacks against guerrillas holed up inside the town.

Independent observers, including foreign diplomats, concluded from the damage that the planes had rocketed buildings to blast out the guerrillas. One projectile hit the Red Cross facility.

Army commanders denied that their planes had done the damage. But Acting Archbishop Arturo Rivera y Damas, the nation's spiritual leader, charged the government with "indiscriminate bombing" and the guerrillas' publicists in Washington announced that "barbaric and indiscriminate bombing" killed or wounded 170 civilians and wrecked 25 percent of the town.

Santiago Yazbek Baltres, Berlin's temporary mayor, later estimated that 20 civilians died in the fighting from all causes. Word that planes had struck Berlin quickly spread through the country. As a result, when guerrilla forces moved into the town of La Palma several days later, villagers repeatedly told correspondents of fears that government planes would bomb their homes.

Reporters who visited La Reina, at one end of El Salvador, and Cacaopera, at the other end, heard expressions of similar fears when guerrilla units were in those towns.

Yazbek Baltres, standing beside the ruins of his brother's house, said he wished neither the guerillas nor the Army had any weapons, "and then maybe we would be left alone."

Salvadoran officers and foreign military sources said pilots demonstrate concern over damage to civilian areas, sometimes returning to base without dropping bombs or firing rockets rather than hitting a risky target. The National Guard has in the past called in air strikes on civilian targets, the foreign sources said, but the regular Army has exercised extreme care.

"You must be pretty precise in the objectives," said Lt. Col. Domingo Monterrosa. "That's a problem."

Since the United States supplied it with A37s, the small Salvadoran Air Force has taken on a more important role in the war. It now flies an average of more than 30 sorties a day, divided evenly between airplanes and helicopters, according to a tally by military officials. Of those, half a dozen are made by the A37 Dragonflies, subsonic jets that carry 500-pound bombs or 2.75-inch rockets in four pods of seven rockets each fixed under the wings.

A few attacks are still made by El Salvador's 1950s-era Ouragan or Fouga jets. But the French-made aircraft purchased from Israel are considered less appropriate for counterinsurgency missions than the slow but nimble Dragonfly.

Other sorties on the tally come from transport craft ferrying supplies or O2 light observation planes called in to pinpoint guerrilla locations for subsequent rocket or bomb runs. In addition, helicopters are in daily use moving men and supplies, and occasionally officers on their way home for the weekend.

The United States supplied El Salvador with six A37s, four O2s and 20 Huey helicopters in an emergency renovation program after guerrillas blew up most of the previous Salvadoran Air Force in a raid on Ilopango airport on the outskirts of the capital Jan. 27 last year. The $55 million aid package, which included pilot training and maintenance help, marked the departure point for the Air Force's increased role in the war.

The value of air power is limited in a conflict where rebels rarely gather in large concentrations or move about in targetable vehicles, the sources said. Bernardo Torres, a guerrilla field commander interviewed in Chalatenango province by Washington Post correspondent Christopher Dickey, agreed with their assessment, saying the lack of a traditional front helps "neutralize" the planes.

El Salvador's 19 remaining helicopters--one crashed--provide troop mobility and a firing platform for .30-caliber machine guns. Several units were helicoptered into Morazan province battle areas during the Army's assault to retake towns there last week. But only about half the helicopters are flying on any given day, military sources said, because of maintenance problems.

Defense Minister Jose Guillermo Garcia has sought more. But U.S. advisers say the Salvadoran capacity to keep them flying is at its limit.

[In the latest fighting, meanwhile, guerrillas have cut the road between San Salvador and Suchitoto, 27 miles to the northeast, and killed at least 25 troops and six civilians, The Associated Press reported. Quoting military sources, the news agency said the attack appeared to be part of a major new hit-and-run offensive by the guerrillas.]