Prodded by U.S. advisers, the Salvadoran Army has launched its most ambitious operation of the 3-year-old civil war to drive leftist guerrillas from this devastated province and remain on hand to protect a long-term reconstruction program.

The effort, "Operation Well-Being," includes what U.S. officials acknowledge is the most pervasive American involvement so far in a Salvadoran military action, with several U.S. advisers posted here to directly supervise what goes on and another dozen rotating in and out for intensified training of Salvadoran units.

The operation, which began Friday, is regarded as a crucial test of the Salvadoran Army's ability to break a pattern of large-scale sweeps followed by withdrawals that allow guerrilla forces to recover their strongholds. As such, it also constitutes a first test for the leadership of Gen. Carlos Eugenio Vides Casanova, who became defense minister seven weeks ago after his predecessor was criticized for sticking with the "sweep-and-run" tactics.

"The troops I have, they are not going to pull them out of here for any other operation in the rest of the country," said the operation commander, Col. Rinaldo Golcher, in an interview here in the provincial capital. "They will be here as long as is necessary."

Golcher's troops, numbering more than 5,000, include U.S.-trained units. In the last four days, they have advanced smoothly behind artillery barrages and bombing runs up the slopes of Chichontepec volcano near here.

Although the volcano has been a guerrilla redoubt for three years, they have encountered only token resistance from the 500 to 1,200 guerrillas of the Farabundo Marti Liberation Front who were believed camped out there.

Government sources have suffered two dead and three wounded since the operation was launched, Golcher said, although only one of the wounded and neither of the dead resulted directly from the new operation. Guerrilla casualties, if any, are not known, he said.

This means most of the guerrillas fled in advance of the well-publicized sweep, following long-established tactics of avoiding unit-to-unit confrontations with the better equipped Salvadoran Army and its U.S.-supplied air power, which includes the A37 Dragonfly, a small counterinsurgency jet used for ground support and UH1H (Huey) helicopters.

"This was an operation announced so far in advance that it was a secret for no one," the colonel said.

The difference this time is that the Salvadoran Army has resolved to keep enough troops on hand long enough to prevent a return of guerrilla forces, providing security for a wide-ranging civic action program. To underscore government assurances on this point, Vides Casanova and several ministers visited here today to preside over a meeting of officials assigned to restore the province's roads, schools, water works, telephones and local administration.

"In this operation, we are going to combine military and development objectives," said Golcher, who was brought here from the command of the Salvadoran Armed Forces Studies Center in the capital 35 miles to the west.

The idea of such a plan had been urged on the Salvadoran Army for more than a year, U.S. officials said. It is broadly patterned on the Civil Operations and Rural Development Support program (CORDS) tried by the United States in Vietnam in the 1960s. Some of the U.S. advisers involved here had experience with the CORDS effort in Vietnam.

Perhaps because the effort in Vietnam became associated with the overall failure of U.S. involvement there, however, American diplomats and military advisers are reluctant to draw the comparisons with what is planned for San Vicente department, or province.

With a U.S. adviser in green fatigues looking on, Golcher said the first civilian operations are expected to begin later this week as his troops secure parts of the province from long-standing guerrilla presence. Behind a long-term military shield, a civilian infrastructure is supposed to resume operations gradually across the area, ending virtual collapse during a guerrilla presence that has turned this farming province from one of El Salvador's most prosperous into one of its most stagnant.

This phase of the operation is expected to be the most difficult.

With his thousands of troops, brand new U.S.-supplied trucks and weapons, and helicopters thumping up and down the mountainside, Golcher has had little difficulty restoring swift Army control over the province. To retain it, however, his men will have to stay in the area and mount the aggressive, repeated patrols that U.S. advisers have been urging without success for months.

Some officials also fear guerrilla forces could stage a large attack elsewhere in El Salvador, probably in the far eastern Morazan province, to force Vides Casanova to order troops away from San Vicente.

In addition, military sources have said the plan eventually will include paramilitary home guards to provide security in villages, and, it is hoped, gradually obviate the need for an extensive Army presence to protect civilian reconstruction. Such local forces in the past have been linked to murders and human-rights abuses that have eroded popular support for the government. Popular support is one of Operation Well-Being's main goals.