Anti-Sandinista guerrillas have been carrying on a new offensive in recent months that has brought fighting to regions that have experienced little consistent combat for about a year and has brought the number of antigovernment fighters operating inside Nicaragua back to 1984 levels.

A Sandinista military officer interviewed here said that in the last two months his troops engaged in 67 battles with the rebels, losing 87 soldiers dead or wounded, while the guerrillas lost 135. He said the rebels staged three ambushes and attacked two agricultural cooperatives and a settlement camp, burning houses.

Guerrilla leaders, analysts in Managua and U.S. State Department sources said the resurgence stems from the recent delivery of supplies to the counterrevolutionaries, or contras, as they are known here, and their determination to prove themselves a viable military force.

"The contras are under the gun to demonstrate that they can get into Nicaragua and do some damage, take on the EPS," or Sandinista Popular Army, said a State Department official who is knowledgeable about the guerrillas. "They're under pressure from Honduras to get into Nicaragua and fight," said the official, who was interviewed by telephone.

The new infiltration of contras has coincided with a congressional debate over whether to provide them $100 million in economic and military aid. It also comes a few months after widespread dissatisfaction about the lack of military activity by the contras surfaced among some of their supporters in Washington.

With the exception of a brief comeback last August, when the contras inflicted heavy casualties during attacks in the provinces of Esteli and Chontales, there was little war activity throughout 1985. The only visibly permanent contra presence has been the Jorge Salazar Regional Command, a force of several thousand in the central provinces of Boaco and Chontales.

"It was quiet most of the last year but now the contras are back in about as much force as in 1984," said the Nicaraguan officer here, who is a ranking military official in northern Jinotega province. He was referring to a lengthy contra offensive during the electoral campaign that autumn.

According to Sandinista military officers in the region and military analysts in Managua, the contras mounted a series of small raids in March and began a heavy infiltration in early April along a two-pronged or three-pronged route running south from the mountainous border regions into the more populated Matagalpa, Jinotega and Chontales provinces.

Sandinista officers estimated that nearly 4,000 contras infiltrated, adding to the already encamped forces of the Jorge Salazar unit. Adolfo Calero, leader of the rebel coalition known as the United Nicaraguan Opposition, said about 15,000 are inside Nicaragua. A U.S. official estimated 8,000 to 10,000.

"We managed to supply our men beginning in February, and now they are fighting," Calero said in a phone interview. "We are on a sustained and continuous struggle," he said, contending that his men do not rely on the base camps known to operate just inside Honduras.

Top Honduran leaders blocked supplies to the anti-Sandinista guerrillas into the beginning of this year. U.S. officials said that since then, almost all of $27 million appropriated as "humanitarian aid" has been spent.

Fighting picked up soon after the first infiltrations in northern regions close to the Honduran border and, more recently, in Boaco and Chontales provinces.

In a May Day speech, Nicaraguan President Daniel Ortega said that combat, ambushes and attacks left 1,811 dead or wounded on both sides in the first four months of this year.

Nicaragua's Defense Ministry is reporting stepped-up ambushes and attacks in towns, farms and resettlement camps, including an ambush last week in Chontales province in which 10 persons, including seven civilians, were killed.

While most of the estimated 1,600 contras recently infiltrated into Jinotega had retreated into more remote regions of the province, said the Sandinista officer here, about 500 were still concentrated as close as 30 miles from the provincial capital of Jinotega.

"Our casualties have been heavy," the officer said from his command post outside of town. "There are contras as close as there," he said, indicating a hilltop several miles away. "We are making them move constantly; they fight for 10 days and then retreat."

He said the Sandinista incursion into Honduras to attack a contra base camp in late March, an attack that was widely publicized by the Reagan administration, was partly aimed at forcing a withdrawal of recently infiltrated contras. He said destroying base camps cuts guerrilla supply lines, demoralizing forces and spurring their retreat. Just after the incursion, he said, several hundred contras retreated into the more remote mountain region.

But contra officials and some U.S. officials said the tactic may have backfired. Those sources said contra military chief Enrique Bermudez's response to the incursion was to send more forces into Nicaragua to prove that the strategy would not succeed in bottling up contras at the base camps.

Costa Rica decided Thursday to grant asylum to Nicaraguan rebel leader Eden Pastora, leader of the Democratic Revolutionary Alliance (ARDE), and about 450 of his fighters, if they come unarmed and under the auspices of the Red Cross, Reuter reported.

[Miguel Carmona, director general of the Costa Rican Red Cross, said the decision was reached at a meeting of government officials. Last Saturday, the ARDE leadership and most of the fighters joined forces, against Pastora's wishes, with the Nicaraguan Democratic Force.]