Government troops have launched their biggest offensive yet in the two-year war with Marxist-led insurgents, putting to the test the massive training, reequipping and attempted reorienting of the Salvadoran Army in which the United States has invested $80 million this year.

Judging from sketchy reports of skirmishing in the rough green mountains of Chalatenango province near the Honduran border north of here, the tradition-bound Army appears to be adapting only reluctantly to the lessons being pressed on it by U.S. advisers.

Reporters have so far been unable to reach the action, and the field commanders are not available for interviews. But one week into the battle, there is little indication that the operation, a hybrid of old and new tactics, has done serious damage to the guerrillas.

There are no specific reports from either side of major confrontations, and the main force of insurgents based in the area, from the faction known as the Popular Liberation Forces, is believed by some analysts to have escaped altogether.

Western observers here are concerned that the expense in manpower and supplies is largely being wasted in Chalatenango, an economically insignificant part of the country where the guerrillas have managed to return relatively unscathed following each previous government offensive, at a time when the U.S. Congress is increasingly reluctant to underwrite continued fighting.

The Chalatenango operation is employing the Pentagon-promoted tactic of using small, mobile bands to confront guerrilla units, according to Salvadoran officers. But some military analysts say it is being used in combination with more traditional methods that negate the advantages sought by the new tactics.

As a result, the mood of optimism around the U.S. Embassy only three weeks ago has turned to frustration and worry that no one wants to express on the record.

Meanwhile, elsewhere in the country, the guerrillas have once again started stepping up their activities.

For weeks there was relative calm as the insurgent factions reportedly tried to patch up their internal conflicts after the political and military setbacks suffered March 28, the day of El Salvador's election, when the small-unit military tactics advocated by the United States helped devastate a sluggish guerrilla offensive.

But in the last few days the rebels have hit government units with a series of brutally effective ambushes, in one of them killing at least seven soldiers on the Pan American Highway.

In the government's Chalatenango offensive, according to military sources, 26 companies, or well over 3,000 men, have been thrown into action to the east of the provincial capital, also called Chalatenango. The government forces include the elite, battle-hard Atlacatl Rapid Reaction Battalion trained a year ago by U.S. Green Berets and, for the first time, another full battalion of young Salvadoran troops trained this year at Ft. Bragg, N.C.

The 960 men in this new unit, named after 19th-century Gen. Ramon Belloso, were trained at a cost of $8.9 million, according to a U.S. Embassy report.

At the same time, several of the 477 Salvadoran officer candidates who recently returned from Ft. Benning, Ga., where their training cost $6.6 million, also are seeing action for the first time.

At least half of the 20 UH1H "Huey" helicopters provided by Washington, many of them part of the $21 million in aircraft replacements made after guerrilla saboteurs devastated the Salvadoran Air Force in January, are being widely used in the operation.

Earlier this week, a soccer field just outside of Chalatenango that acts as a command post was the scene of constant helicopter traffic as old women who cook for the troops were carried off to forward villages among sacks of potatoes, and stacks of 250-pound bombs were unloaded from hovering choppers coming in from the the capital.

The new techniques are supposed to use the highly trained infantry units in small, mobile squads to hunt and kill guerrillas by using guerrilla-style tactics.

That was the point of much of the training in the United States since, until recently, the insurgents moved through El Salvador's rugged farmlands, especially at night, with almost total impunity. The advantage of surprise was always theirs.

"You have to take the night and the countryside away from them," said one senior Western official concerned with military strategy here. Squads of about a dozen men can be used to "saturate" areas where guerrillas operate regularly so that "they run into us instead of our stumbling along banging into them," the official said.

"We're taking the war to them," said a U.S. colonel who worked with Montagnard tribesmen in Vietnam.

While the use of such techniques in that Southeast Asian war and later in Angola did not affect the final outcomes, variations on this general approach were considered successful against insurgencies in Malaysia, the Philippines and Venezuela, and in Bolivia against the revolutionary Che Guevara, according to one senior Western observer.

These methods could be used to help establish and guarantee the security of productive areas of the country, and, according to well-informed Western observers, that is the basic strategy promoted by U.S. military advisers here in response to the most effective tool employed by the guerrillas so far: economic sabotage.

But most of the sabotage is being conducted far from Chalatenango, one of the most backward of El Salvador's 14 provinces and a region with a heavy concentration of guerrilla base camps where the insurgents generally have an advantage both in familiarity with the terrain and support of the population.

According to Salvadoran officers informed about the operation, the basic technique being used in Chalatenango is directed toward trapping the guerrillas and making them fight. Most troops are used for blocking, in this case ostensibly cutting off escape routes across the Honduran border.

This is the same basic technique that has been tried with very little success for most of the war. But now small units are being employed, as the Pentagon had hoped, to move quickly and quietly against guerrilla positions, rather than in full platoons of more than 40 men that telegraphed their every move as they tramped through the rough terrain.

As military analysts here point out, however, the problem with this combination approach is that while the elite troops operating in small squads may be able to carry out the push, the more conventional units used for the major blocking maneuvers have rarely proved effective.