The Nicaraguan Army, in five days of sporadic combat ending last week, has broken up what it describes as a major attempt by U.S.-backed counterrevolutionary guerrillas to occupy part of Nicaragua and declare it a liberated zone.

The fighting 135 miles north of Managua involved mortar exchanges, artillery fire from Army gunners and the first open deployment of Managua's Soviet-made T55 tanks and BTR60 armored personnel carriers since the guerrillas stepped-up attacks against the three-year-old Sandinista government last summer, Army officers said.

It came in response to what Sandinista commanders here portrayed as coordinated approaches by three counterrevolutionary units totaling 900 men. This would be an unusually large force in what heretofore has been a series of harassment raids and sabotage by commando teams from hideaways inside Nicaragua and across the border in Honduras.

The State Department has refused to comment on widespread reports that the Reagan administration is providing direct and indirect assistance to the anti-Sandinista forces, saying that is it against policy to comment on intelligence matters.

The Jalapa regional commander, Capt. Rodrigo Gonzalez Garcia, said one group of about 250 men was discovered moving in from the east near the Arenal de Yali River. A second group of 500 was encountered moving west in the rugged border hills of Jesus, he said, and a third of 150 was found farther south, well inside Nicaragua, with the apparent mission of cutting off the region's only all-weather road linking Jalapa with the rest of the country.

Their aim, Gonzalez said, was to take Jalapa and make it the capital of a liberated zone in this northernmost salient of Nueva Segovia province. Although Jalapa has only 9,000 inhabitants and the area comprises mostly hillside coffee farms, occupying even a small piece of Nicaraguan territory would be a major advance for the anti-Sandinista guerrillas.

Gonzalez declared that all but a few dispersed bands were driven back into Honduras in five days of fighting that ended Tuesday, proving, in his eyes, that they are unable to remain inside Nicaragua. But he acknowledged that part of one unit came as close as two miles from Jalapa before being discovered and forced back in three days of shooting at the village of Santa Fe.

This means the guerrillas were able to advance four miles from the border hills undetected. A Sandinista officer said his men discovered more than 1,500 yards of trenches in the hills near Cerro el Aguila, suggesting the counterrevolutionaries had been inside Nicaragua some time before making their move.

A half dozen mules and ponies were seen wandering around mountain trails just under Cerro el Aguila, left behind by the retreating guerrillas. Sandinista officers said the pack animals had been used to bring guerrilla supplies across the steep slopes from Honduras, including 60 mm mortars, M60 machine guns, ammunition and food.

The counterrevolutionary units lost more than 30 men in the clashes that led to the dispersal, Gonzalez said. His own forces suffered seven killed, including two officers, and twice that many wounded, he added.

The Sandinista Army used artillery and 81 mm mortars to dislodge some guerrilla positions, an officer said. Although a few T55 tanks and BTR60 armored vehicles were deployed around Jalapa to protect it from attack, they were not used in combat, according to the Army spokesman, Capt. Roberto Sanchez.

Their deployment marked a change in Sandinista policy, however, since previously the tanks and APCs had been kept hidden from view at sites near Managua. One BTR60 seen parked at Army headquarters here was missing two of its eight wheels, apparently disabled in the rough terrain.

Foreign Minister Miguel D'Escoto dispatched two protest notes to the Honduran government Thursday, charging that the guerrillas had come from and retreated back to Honduran territory despite pledges from President Roberto Suazo Cordova's government that they would be controlled. D'Escoto named what he said were several guerrilla camps inside Honduras and added that the Sandinista leadership "is running out of patience."

Some counterrevolutionary camps along the border were shut down about six weeks ago by the Honduran Army. Honduran military sources and Nicaraguans involved with the guerrillas said some of the units had gone to new bases inside Nicaragua while others had been taken to sites within Honduras for more training.

The latest fighting suggests the Honduran Army's decision to relocate camps amounted to an effort to gain more control and keep guerrilla activities out of public view rather than a move to halt cross-border movements.