The huge propellers can be heard cutting the air in the early evening, the heavy vibrations sending townspeople into the dusty streets to search for "the phantom plane" against the distant stars and sliver of a moon. Jittery troops in the garrison and on the bridges leading into town fire off rounds from their M16s at the unlit target.

Residents say the plane is going north to resupply the several thousand guerrillas who control the northern region of this province of Morazan.

Maj. Carlos Lenarez, an officer in the Salvadoran Army garrison here, said that Army troops control the province and that "the guerrillas do not even come out to fight." But military officials in the capital, San Salvador, say that as many as 4,000 troops now are being sent to Morazan province to reclaim a growing number of the surrounding towns and villages occupied during an ongoing guerrilla offensive and to push back the rebel advance toward this provincial capital.

Even in "normal" times, the Army appears to have a difficult task in this barren region, known as the "Siberia of El Salvador." Commanders seem to change every few months and Army helicopters, fearful of guerrilla fire, no longer fly over the area.

The 300 special commandos here, who model themselves after the U.S. Green Berets, regularly put on a show for the local residents, most of whom are civilian employes of the Army. They run around the town plaza with dead vultures in their mouths or tromp through the streets shouting, "The commando never dies," but they rarely make forays into the surrounding countryside. When they do leave the barracks it is usually to suffer a disastrous defeat by the guerrilla forces, who control much of the surrounding countryside.

The Army prefers to hold its garrison and the movie theater and municipal swimming pool, which it expropriated from the town, rather than deploy troops. "The subversives are trying to draw us out," said one recruit on guard duty outside the garrison, "so they can take the town."

A traveler to the nearby villages of Yamabal and Guatajiagua says goodbye to Army control at the small bridge on the edge of town. The guerrillas send out patrols that nonchalantly saunter down the center of the dirt road connecting the two towns, barely five miles outside San Francisco Gotera.

Until a few months ago, civilian guards working for the regular Army used to stand duty in Yamabal and Guatajiagua. But then, residents said, several dozen guerrillas dressed in Salvadoran Army uniforms entered Yamabal, demanded to speak to the 14 local guards and, when they reported, gunned them down inside a house.

When the town of Guatajiagua was taken at the end of December, the guerrillas were a bit more judicious. Three civil guards reportedly died in combat, two were taken prisoner, and one was released.

In Guatajiagua, a town of 4,000, red-painted slogans cover the adobe walls. The various sayings read like one-liners from a revolutionary primer, such as "To be uninformed is to be disarmed." No one in the town seems happy about the desecration of the walls, but the slogans remain.

A red flag hangs from the town municipal building, placed there by members of the People's Revolutionary Army, one of the five guerrilla groups that make up the Farabundo Marti National Liberation Front.

Two guerrillas on patrol move about the town in straw hats, carrying M16s. They wear ammunition belts and politely greet the town residents who pass.

"The people are very nervous," said one of the guerrillas, a woman in her twenties. "There are many spies who report to the Army those who befriend us, so most of the people do not speak." She and a teen-age companion were both dressed in neat, clean civilian clothes.

The woman, although nervous, was talkative. She called herself Liset, admitting that it is not her real name. She said she has been fighting for four years and is married to another guerrilla, although the war has made it impossible for them to live together. They have one child.

"We are fighting two wars," she said. "We fight the war with the Army and we fight the war to educate the people. Many of them do not want to see the repression around them. They do not want to believe that life can be better."

According to these two guerrillas, the Army has made attempts in the Morazan province to use guerrilla tactics. "But the patrols they send into the field are demoralized and easily taken," said Liset.

"In the middle of January, we took the town of Osicala. The Army sent many troops to retake the town. We went into the surrounding areas and harassed them until they packed up their food and supplies and ran back to the garrison." The insurgent forces claim to control 18 of the 26 towns in this province.

In Guatajiagua, life continues unimpeded by the People's Revolutionary Army, whose forces pay for items from the market and have, with the exception of the civil guards, left the populace alone. The guerrillas held one town meeting to explain their political doctrine. A Salvadoran priest living with the guerrillas offered a mass.

Despite the low profile by the insurgent forces, the townspeople are nervous. The phone lines have been cut, and supplies from the city of San Miguel no longer arrive.

"I know the people here fear a fight," Liset says. "They think we will not protect them."

"I stay most of the day in my home," said an 18-year-old woman who is a maker of the red earthenware pottery for which the town is known. "I wait for the night when the mortars start falling and the Army comes back."

"When this happens," said another villager, "it is we who suffer, caught between two lines of fire."

A pineapple vendor said, "It is not the guerrillas who scare us, but the war. We have lived quietly; now we all wonder when the fighting will come."