CHALATENANGO, EL SALVADOR -- Attacks by leftist rebels that started last month have provoked the heaviest fighting outside the capital in years, causing 400 deaths, incluing those of 25 civilians caught in rebel assaults on military bases in populated areas.

The rebels have fought in larger units than in the past and have used antiaircraft missiles. Guerrillas have shot down two planes -- by using the missiles -- and a helicopter, and forced two other copters to make emergency landings, in one case killing a pilot and a gunner.

The rebels deny that their latest actions are an offensive, in deference to the recent halving by Congress of U.S. military aid to El Salvador. That aid could be restored if a rebel offensive were determined to be underway.

The Salvadoran response to the attacks by the Farabundo Marti National Liberation Front (FMLN) was to move the 800-man Atlacatl Battalion, supported by other units, into northern Chalatenango province just before Christmas for a 45-day operation.

"The guerrillas are now massing their forces to hit individual army units hard," said Capt. Jose Gallardo of the Atlacatl Battalion as he examined through binoculars the steep mountain peaks across the valley. "They have reverted to the tactics of earlier in the decade. So now we must not fall into the error of diluting our forces. We have to go in strong."

The battalion advanced like a conventional army, in four 200-man prongs spread over several miles. They stuck to the hilltops, supporting each advance with heavy weapons. The soldiers, laden with four days of canned rations, ammunition, heavy machine-guns and mortars, sweated up the mountainsides through shoulder-high grass.

From the third day onward, skirmishes broke out every few hours as the rebels harried units at the edges of the army's advance. The first warning of a rebel ambush is the crack and whiz of bullets flying overhead. As the soldiers move forward against their unseen enemy, they shout and scream insults at the guerrillas.

"If they are nervous it gives them more courage if they can shout and shoot their weapons. They feel more sure of themselves hearing the firepower we have," said 2nd Lt. Jose Luis Sol.

In two firefights witnessed by reporters, officers estimated the troops had fired 4,000 M-16 automatic rifle rounds, 1,100 heavy machine-gun rounds, 80 rifle grenades and an antitank weapon. The only known casualty was one soldier with a bullet wound in the leg.

The rebel campaign in the countryside comes at a time when the army is under increasing political pressure. In October, angered at the lack of progress in the investigation of six Jesuit priests, the U.S. Congress cut half of El Salvador's $85 million military aid.

The Jesuit massacre was carried out by troops from the Atlacatl Battalion, which as well as being the army's elite fighting unit, also has one of the most criticized human rights records. Rights groups attribute to it several massacres of peasants earlier in the war.

Now the army's policy in northern Chalatenango is to avoid moving into civilian settlements where many of the people support the rebels. "It is hard. The people down there hate us," said one soldier, who said he was afraid to buy tortillas from civilians for fear they might be poisoned.

Morale in the Atlacatl, which unlike other army units has suffered relatively few casualties this year, is high. Its firepower remains heavier than that of the guerrillas. But the soldiers clearly did not understand why U.S. aid has been reduced. The cut, combined with the lack of air support, is taking its toll.

"The guerrillas are well prepared and motivated. They have good weapons," said Marvin Alcedes, 22, a six-year veteran. "This thing isn't going to end. There is too much hatred and vengeance."

Like most soldiers, Alcedes said he was originally forced into service. But he volunteered to stay on because he believed he could help win the war. Now he says he wants to leave the army and find a job in Los Angeles.

Conditions for the soldiers are hard. They move in the heat of the day. In the dry season, water is hard to find and thirst is constant. It is the rebels who decide when and where to fight.

Because the government is providing less air support for its troops, the rebels can mass into much larger units to attack. Army wounded have to be carried out of danger on the shoulders of their comrades.

There is a constant fear of snipers, mines and booby traps. As a 20-man patrol struggled to carry a wounded soldier to safety over rough terrain, the soldiers talked about friends who had been maimed.

"A friend stepped on a mine earlier this year," said one soldier. "He was the one who always laughed and joked. But from the day they amputated he was all serious. He wouldn't talk to anyone. He only talked about marrying his girlfriend but I told him she would be with someone else. They cut off both his legs."

The rebels in the area, who appear to have a more flexible command and control structure, say they are not impressed by the large-unit army tactic -- which they are easily able to avoid by hiding.

"They don't cause any problems for us," said a one-armed rebel fighter called Tito, interviewed last month in the nearby town of San Jose las Flores. "All they are doing is guarding hilltops."

Rebel leaders say the aim of the military campaign, which they warn will continue through coming months, is to force concessions in U.N.-sponsored peace talks. Negotiations have been stuck over rebel demands for a sweeping purge of the army officer corps and an eventual demilitarization of the country.

The rebels face a high political cost for the campaign, especially from civilian casualties caused by the fighting. The government has been running a television and newspaper advertising campaign claiming the attacks demonstrate the rebels are not serious about peace.

State Department officials have called the introduction of missiles to the war a "significant escalation." The State Department has said the Soviet Foreign Ministry confirmed the number on a discarded firing tube of a SA-14 missile found in El Salvador last month to be that of a missile delivered to Nicaragua in 1986.

The State Department said it would speed the delivery of the $48.1 million of military aid which has been approved by Congress. Military sources say the money will be used to replace downed aircraft.