The Sandinista government charged today that U.S.-sponsored counterrevolutionary guerrillas have stepped up attacks on Nicaragua sharply in the past week, including "terrorist actions" against bridges that seem to signal a significant shift in rebel tactics.

The "new escalation" reported by the Defense Ministry ended six weeks of relative calm in the year-old conflict between Sandinista forces and Nicaraguan rebels headquartered in Honduras and supported by the Honduran Army and the U.S. Central Intelligence Agency.

Several targets listed by the ministry were deep inside the country, including a bridge at El Tuma 13 miles northeast of the city of Matagalpa. This seemed to bear out reports from Sandinista officials in recent days that the rebels may be putting aside attempts to capture and hold territory in the rough border hills in favor of deep-penetration commando raids.

Hugo Torres, political chief of the Popular Sandinista Army, said Monday that combat also has increased in the steamy stretches of Zelaya province in recent days and that Sandinista intelligence from Honduras indicates "a large-scale invasion" could be under preparation for that remote area. Nicaraguan military sources said they believe the rebels have begun using helicopters to carry in commando teams and resupply their units inside the country.

Jorge Ignacio Ramirez Zelaya, identified by the Army as a captured commando leader, has told interrogators that the guerrillas have two U.S.-made UH1H "Huey" helicopters and a C47 transport plane at their disposal in Honduras, along with a pair of light aircraft, according to official press reports. Hueys and C47s, both in the Honduran Air Force inventory, are regular sights in the military section of Tegucigalpa's international airport.

Use of such aircraft would expand the scope of the conflict here. Previous attacks have been circumscribed by the rebels' need to walk long distances over rough terrain southward from the Honduran border, risking a chance meeting with frequent Sandinista militia patrols.

During visits to Washington earlier this summer, political leaders of the Nicaraguan Democratic Force, the largest counterrevolutionary group, sought increased U.S. logistical support for their estimated 7,000 troops. The leaders said then that unless they could move their efforts farther inside Nicaragua, they and the Reagan administration would risk further erosion of U.S. congressional willingness to consider them a legitimate challenge to the Sandinistas.

According to the force's leaders, their one significant attempt to infiltrate deep inside the country, and spark what they hoped would be an anti-Sandinista popular insurrection, failed in February, when their fighters were stranded in Matagalpa without promised supplies.

The geographical expansion of the Democratic Force's activities appears to coincide with the shift to traditional guerrilla tactics, such as small-unit infiltration and economic sabotage. Perhaps because it is commanded by former officers of the now-defunct Nicaraguan National Guard, its tactics previously have been more like those of an Army.

A prolonged attempt in June to hold onto the captured hamlet of El Porvenir near the Honduran border--using large rebel units and relatively heavy weapons--ultimately led to a rebel retreat. This may have precipitated some rethinking, according to speculation by Sandinista officials.

Lt. Cmdr. Roberto Sanchez, the Army spokesman, said a commando team tried to destroy the El Tuma bridge last Friday with plastic explosives, damaging parts of the structure and forcing authorities to bar passage of heavy loads. Sandinista militia forces caught the guerrilla unit four miles to the east, killing nine and taking three wounded of their own, a Defense Ministry communique said.

The bridge lies about 80 miles north of Managua and more than 120 miles south of the border. Two days later, the regular Sandinista Army clashed with a second antigovernment unit in the nearby Matagalpa province town of Paiwas, killing six and suffering six killed, the communique reported.

A specialized team hit another bridge Monday at El Jicate on the Yali River, just south of the border in Nueva Segovia province, it added, but was driven off before doing any damage. Reported casualties were one Sandinista reservist and five guerrillas dead.

The largest battle reported by the ministry took place yesterday in Nueva Segovia's Ciudad Sandino in the north. More than 200 irregulars attacked in the early morning but were driven off in fighting that killed two members of local defense forces and 21 guerrillas, the communique reported. In all, it said, 87 guerrillas and 13 Sandinistas were killed in the week's intensified combat.

Interior Minister Tomas Borge told visiting American newspaper editors this week that about 1,000 Nicaraguans, counting Sandinistas and rebels, have been killed since the beginning of the year. Although estimates vary widely even among official sources, and although the Sandinistas often report casualties lopsidedly in their favor, by all accounts the toll is said to include at least several hundred soldiers and reservists.

The reported escalation in fighting coincided with U.S.-Honduran exercises on the other side of the border. The Sandinista leadership at the time of U.S.-Honduran exercises last fall charged that stepped-up military activity was being used to screen mobilization of Nicaraguan guerrilla units and transportation of supplies to border dumps for subsequent use by irregulars crossing into Nicaragua.