The Salvadoran Army launched a major offensive today against the left-wing guerrilla stronghold in northeastern Morazan province, three days after unprecedented peace talks between the government and the insurgents.

The offensive here came one day after the Army began operations against two smaller rebel-controlled areas in southern Usulutan and northern San Vicente provinces, Armed Forces Chief of Staff Col. Adolfo Blandon said. The offensive in Morazan was "very important," he said, because the Army intends for the first time to keep troops north of the Torola River, which has been the border between government- and rebel-dominated territory since late 1982.

Blandon stood this morning with Col. James Steele, chief of U.S. military trainers in El Salvador, on the edge of a grassy field here and watched 12 UH1H "Huey" helicopters leave to ferry 800 troops north to the towns of San Fernando and Perquin deep in rebel-dominated territory. Initial reports said that they met no resistance.

An additional 1,500 troops were marching north into Morazan from five separate points, Salvadoran officers said. About 500 were seen wading the thigh-deep Torola carrying automatic rifles, recoilless rifles, machine guns and heavy packs just north of this small town.

"In the first place, the objective is the search for and dislodging of the terrorists in the entire zone" north of the Torola, Blandon said. He said that the timing of the offensive had "no relationship" with Monday's peace talks, and that it was part of a strategic plan hatched in July for the second half of the year.

But Lt. Col. Domingo Monterrosa, field commander for the Morazan operation, said he hoped to surprise guerrilla commanders in the middle of talks among themselves over the Monday meeting between the left and President Jose Napoleon Duarte.

"If Monday they met in La Palma, it's probable that people from there are here trying to talk with rebel leaders , and explain to them what happened in La Palma," Monterrosa said.

Northern Morazan is the stronghold of the People's Revolutionary Army led by Joaquin Villalobos. Villalobos was supposed to participate in Monday's talks but failed to show up because he was unable to travel there in time.

Monterrosa said that he had hoped to start the offensive Tuesday, but the Air Force was not available because it had been on alert in case of trouble related to the La Palma meeting.

Two A37 jet fighter-bombers streaked south over this town just south of the Torola this morning. Minutes earlier a loud explosion had echoed in from the mountains to the north where the guerrillas hide out.

"No truce has been discussed," Monterrosa said. "There are times when you have to make war to achieve peace."

The operation here marked the first time that the Army has pushed north of the Torola since August, Monterrosa said, and it marked the first time that the Army used helicopters to carry large numbers of troops north of the Torola.

The delivery last month of 10 additional Huey helicopters, supplied by the United States, made the operation possible at this time, Blandon said. The Army now has 28 combat Hueys, plus four for use as medical evacuation helicopters.

"In the past, when the Army moved north, the guerrillas would see them and retreat. Here you have a significant advantage because you can lift the troops in behind," a military observer said.

U.S. adviser Steele, wearing camouflaged fatigues and carrying a semiautomatic rifle, said he had been invited by Monterrosa to watch the troops climb into the helicopters.

Two other U.S. military trainers also were here: Marine Lt. Col. David Blizzard, who is the U.S. naval attache, and a sergeant who is an aide to Steele. They also were carrying small semiautomatic rifles.

This journalist and five others, all from the United States, met Army troops crossing the Torola River going north as we crossed it headed south this morning after having tried to interview Villalobos.

We met a guerrilla roadblock near Meanguera yesterday and spent the night there while a message was sent requesting the interview.

This morning a guerrilla courier told us that "the leaders" had decided that we could not enter rebel-dominated territory because "they cannot interrupt what they are doing."

We began walking south early this morning, back into government-controlled territory, and we met the troops crossing the Torola.

Blandon said that between 6,000 and 7,000 troops were involved in five current Army operations against the rebels. They included operations in Morazan, Usulutan and San Vicente that began in the past 24 hours, plus one that opened a week ago in central Cabanas province and a fifth that has been under way for months in northern San Miguel province.

The Army's continued presence in northern San Miguel, which borders Morazan, has disrupted communications between the rebels' two principal strongholds in Morazan and Chalatenango provinces to the west.

Blandon and Monterrosa stressed that they believe the guerrillas are weak right now and noted that the Army is using fewer troops in the offensive against Morazan than in offensives last summer here.

In June, Monterrosa drove into Morazan with about 6,000 troops and briefly occupied Perquin, considered the "capital" of the rebel-dominated part of the province.

"They are weak militarily. They are weak logistically. They are weak popularly . . . , politically . . . , internationally," Blandon said.

The chief of staff also said that the Army intended to transfer some civilian refugees back to the area north of the Torola and provide them with security and government services in an effort to begin pacifying the province.

This effort would begin today in Meanguera, where a group of refugees from the area near the town was set to return today.

The Army also was hoping that local civilians using government materials would be able to begin rebuilding a bridge over the Torola that rebels destroyed in 1982.

The Salvadoran Army has requested increased U.S. military aid for this program, and a military source said that private support from the United States might be arranged to sidestep restrictions on use by the Salvadoran military of U.S. Agency for International Development assistance.

In particular, a plan was being considered for U.S. National Guard units to "sponsor" Salvadoran Army units by donating used clothing, medicine or other aid, the military source said.

Monterrosa said that the Army knew that the guerrillas were weak in part because of information provided by two Salvadoran officers who recently were returned by the guerrillas in a prisoner exchange. These officers had been held for some time in northern Morazan province.

Morale among the Salvadoran troops appeared to be high. As Monterrosa drove by in a jeep, soldiers chanted and shook their fists in the air or pumped their rifles up and down.