Leftist guerrillas, engaged in the most serious battles against government troops this year, have shot down their first U.S.-made UH1H "Huey" helicopter in sharp fighting which is inflicting punishing losses in both men and material on the increasingly harried Salvadoran armed forces.

Defense Ministry officials today confirmed that the helicopter was shot down Monday during operations in the mountainous northern part of Morazan Province here during major military operations in the area. The helicopter was one of 10 supplied to El Salvador by the United States earlier this year. The defense spokesman said its Salvadoran crew escaped unharmed and said that none of the U.S. advisers training Salvadoran pilots had been anywhere near the area.

The Defense Ministry spokesman said that the Huey downed Monday was hit by guerrilla machine-gun fire on the precipitous slopes of the 3,100-foot-high Cerro Buena Vista, 20 miles north of here. The area is one where the government has been trying, with little apparent success, to root out well-entrenched, and increasingly effective, guerrilla units, in a series of sweeping operations that have lasted more than a month.

The government said the aircraft had not crashed but had made a "forced landing."

Aside from the downing of the Huey, more than a dozen trucks and jeeps reportedly have been knocked out by the guerrillas in hit-and-run ambushes. Military casualties have also reached serious proportions, running as high as 20 a day in recent weeks, according to one well-informed source.

Conversation and observations during a two-day visit to Morazan Province, where the guerrillas have long been active, indicate that the fighting now going on in the volcanic and wild northern part of the province is not going well for the government forces.

Although the official line here in the capital remains full of confident pronouncements that since their aborted "final offensive" in January the "subversives" in the hills are a spent force with waning popular support, the picture in San Francisco Gotera, the capital of Morazan Province, is very much the opposite.

A once sleepy market town for the impoverished peasants in the surrounding mountains, this run-down city of potholed streets and white plaster walls has been transformed into a bustling command center for the war that begins just beyond its city limits.

Because of a recent influx of reinforcements from elsewhere in the country in recent weeks, the town's movie house and city hall have been converted into makeshift barracks.

The square itself functions as a helipad where helicopters from the fighting in the north, with twin 50-caliber machine guns poking out of their sides, come and go during the day to disgorge wounded soldiers for transportation to the local hospital. There four Army doctors from the military hospital in San Salvador have been brought in to augment the normal staff of six civilian doctors, and new X-ray equipment and a blood bank have been installed.

"You have to understand that there is a constant state of war here," a young civil servant tells a visitor after quietly begging that his name not be mentioned lest he wake up one morning with "a bullet in the head for talking to you."

The state of war is officially confirmed by the commando barracks commander, a haggard and unsmiling young colonel named Cisneros.

"There are subversives all around out there," he tells a visitor impatiently as he looks up from his command jeep in the square at the jagged volcanic hills that loom over the town. "There are a lot of them. All the roads to the north are dangerous. There are mines and ambushes."

Indeed, as he spoke, it transpired that there was a two-hour fire fight between his troops and guerrillas at a cemetery astride the highway 5 miles to the north. It was just one of numerous battles that were raging in the mountains.

By far the biggest area of operations is around the town of Rosario, north of the Torola River and some miles south of the Cerro Buena Vista where the Huey was forced down Monday. The guerrillas took the town April 16 and for two weeks held off government attempts to retake it.

When the government finally mobilized a force of over 2,000 men, many from units transferred to Morazan from garrisons around the country, it surrounded Rosario in what was supposed to be a major military operation. Well-informed local sources in Morazan say that on the night of May 1, before the government could attack, the guerrillas slipped through their lines unscathed.

The Rosario operation was part of a larger government effort to sweep through the rugged northern corner of Morazan to capture guerrilla base areas and supplies. It is an operation like the one that Western military analysts here recall the government conducted last October, only to have the guerrillas reinfiltrate the area when the government troops moved on.

So far there have been few reports of guerrilla casualties, a fact observers say indicates they have mastered the basis of hit-and-run tactics.

The government so far also has refrained from reporting many military casualties, but the picture one gets in Gotera from watching the helicopters being in the wounded and from listening to reports of overflowing hospital wards is confirmation that they are high.

"All I can tell you," says one Western military analyst in the capital with close ties to the Salvadoran armed forces, "is that for the size of their Army they are taking a lot of casualties."