Lt. Col. Domingo Monterrosa, the Salvadoran Army's top field commander, and two other key field officers were killed today in guerrilla-infested Morazan province when the helicopter in which they were flying crashed during a major Army drive, a military spokesman said.

The armed forces said it appeared that the helicopter had crashed because of a "mechanical failure," but the guerrilla radio station based in Morazan said the aircraft had been shot down.

The death of Monterrosa and the other field commanders -- all of whom had drawn praise from U.S. officials -- marked the biggest setback for the armed forces since the rebels' half-day takeover of the Cerron Grande dam and hydroelectric plant in June. It occurred at a time when the Army had regained the initiative in the field, and commanders were growing increasingly confident.

"This is a major setback for El Salvador," a U.S. Embassy official who declined to be named said tonight. "It happened just when things seemed to be going well."

Lt. Col. Ricardo Cienfuegos, the Salvadoran armed forces' official spokesman, said tonight it was "almost certain" the crash was caused by "mechanical failure." But Radio Venceremos, the guerrilla radio station based in Morazan province, said in a communique signed by guerrilla leader Joaquin Villalobos that the helicopter had been shot down with a heavy machine gun.

Killed with Monterrosa, the commander of all military operations in the country's eastern three provinces, were Lt. Col. Napoleon Calitto, the commander of the Morazan province garrison, and Maj. Armando Azmitia, the commander of the crack Atlacatl Battalion, the U.S.-trained rapid-deployment unit that Monterrosa had commanded until November 1983 when he was elevated to his regional command. The three officers, and at least 10 other persons riding in the helicopter were killed on the sixth day of a major offensive that Monterrosa had launched against rebel strongholds in the province in northeastern El Salvador.

"There are no survivors," Cienfuegos said.

Using 2,300 crack troops, Monterrosa invaded the northern part of the province above the Torola River that had served as a base area for about 2,000 guerrillas, only three days after President Jose Napoleon Duarte held a historic meeting with rebel leaders to try to get peace talks going to end their five-year-old rebellion.

Shortly after launching his operation, Monterrosa had told western journalists that although peace talks had begun there had been no cease-fire yet. He said his drive was part of a government strategy of talking peace and fighting at the same time.

Until today's setback the Army operation into northern Morazan, which had been dominated by the guerrillas for most of the past three years, had been going well, according to Monterrosa's assessment just before his death.

"This is a new type of operation, and we are successfully carrying the fight to the subversives," the 43-year-old lieutenant colonel had said in an interview at his headquarters in the eastern town of San Miguel Sunday. "We are using one third of the men of past operations and gaining more territory than ever."

It was during the launching of this operation last Thursday against the main rebel "capital" of Perquin that three U.S. military advisers traveling with Monterrosa were spotted in an apparent contravention of U.S. guidelines banning advisers from traveling to any area where combat is considered a possibility.

U.S. Ambassador Thomas Pickering had defended the presence of Army Col. James Steele, the head of the U.S. military adviser team in El Salvador, and two other advisers in Perquin by asserting that they faced no danger because they were in the heart of a government battalion.

Pickering told journalists late last week that the U.S. advisers had only accompanied Monterrosa to Perquin after the lieutenant colonel had personally assured them that they would face no danger with him.

Pickering said the U.S. advisers with Monterrosa were "as safe" with him as they would be in any other place in El Salvador.

Monterrosa, who was trained at Fort Benning, Ga., in 1982, along with the Atlacatl Battalion, was considered the leading officer of what U.S. officials portray as the new generation of professional leaders in the Salvadoran armed forces.

U.S. officials repeatedly identified Monterrosa as one of the brightest and most effective commanders, the sort of man who inspired his units to previously unheard of military success.

The "tragic accident" occurred at approximately 5 p.m. near the town of Joateca, 75 miles northeast of San Salvador, Cienfuegos said at a hastily called news conference at armed forces headquarters tonight.

The armed forces immediately announced replacements for Monterrosa, Azmitia and Calitto. Replacing Monterrosa as overall commander in the east is Lt. Col. Miguel Mendez, who until now had been the Army's chief of operations. U.S. officials have praised Mendez for quiet but professional work in that capacity.