Nicaraguan antiaircraft fire hit two of three helicopters that attacked a Nicaraguan military observation post on the Honduran border Monday, the Nicaraguan government said today.

Military spokesman Lt. Guillermo Gonzalez said the helicopters went down in Honduran territory, but he was uncertain whether they had crashed.

"According to information that we have received, they were damaged . . . by gunfire," he said in a telephone interview from Managua.

In Washington, Nicaraguan Embassy spokeswoman Sarali Porta said the two helicopters, presumed to belong to antigovernment forces, had gone down within 600 yards of the border, and the Sandinista government had offered to fly foreign correspondents to the border to view the wreckage. She said the third helicopter was also visible about a mile and a half from the border but said she did not know if it had crashed or landed normally.

A Pentagon spokesman said that the report has not been confirmed, and the Pentagon has no information about the nationality of the helicopters.

The attack in northern Nueva Segovia province had been reported earlier by Nicaragua, but it was not reported that helicopters had been hit.

The Nicaraguan Foreign Ministry sent a formal note to Honduras on Monday protesting that three unmarked military-type helicopters had penetrated Nicaraguan air space from Honduras in the Arenales area, where Nicaraguan rebels have been active. The note said that the helicopters crossed the border at 11:12 a.m., attacked the observation post and were repelled by antiaircraft fire.

The Honduran Embassy in Washington released the text of a note to Nicaragua denying that any unidentified helicopters had flown from Honduras to Nicaragua. "There are no aircraft in Honduras without the correct identifying insignia, and therefore (the) government rejects the note," said the Honduran statement.

There have been reports recently that the Nicaraguan Democratic Force, the Honduras-based anti-Sandinista guerrilla organization, possesses several helicopters as part of its small air force.

But there was no indictation that the helicopters that were involved Monday were from this force.

The incident occurred at a time when a Nicaraguan military offensive against the rebels, known as contras or counterrevolutionaries, has led to stepped-up incidents on the borders with both Honduras and Costa Rica, where the rebel groups are based.

President Reagan has been lobbying hard to get Congress to reverse or modify its vote last April to deny aid to the anti-Sandinista forces. The United States had supplied them with about $80 million in aid until it was cut off about a year ago.

News services reported the following:

Nicaraguan Air Force planes bombed a rebel airstrip and attacked guerrilla positions along the San Juan River near the Costa Rican border in the second week of a major offensive, Nicaraguan military sources told Reuter in Managua.

They said Tuesday's bombing of the 2,500-foot airstrip known as La Penca, near the town of El Castillo on the bank of the river that marks part of Nicaragua's southern border with Costa Rica, severely damaged the facility operated by the rebels of the Revolutionary Democratic Alliance.

It was the second round of bombing in three days.

The sources in Managua said helicopters ferried troops to mountain clearings in the jungles along the river, where they fired artillery rounds at rebel camps in the area, forcing the rebels to flee into Costa Rica. No casualty figures were released.

The operations in the south, launched on May 26, are part of an extensive antiguerrilla campaign aimed at wiping out rebels in the jungles bordering Costa Rica and in the mountains along the Honduran frontier in the north.

In San Jose, Reuter reported, Costa Rican Public Security Minister Benjamin Piza said his country had asked the United States to speed up a shipment of arms and military equipment for its poorly equipped Civil Guard in case of further attacks.

The Associated Press reported from Managua that Canadian Foreign Relations Minister Monique Vezina signed an $11 million loan agreement allowing the Sandinista government to move ahead on a major geothermal electric project.

The Momotombo plant, harnessing power generated by the steam from a volcano in Leon province northwest of the capital, opened last year and is the largest such operation in Central America.

Vezina said at the signing ceremony that the economic cooperation agreement "constitutes for us a contribution to development and peace."

Nicaragua moved its foreign trade office from Miami to Toronto after Reagan announced May 1 an embargo on U.S.-Nicaraguan commerce.

In Tegucigalpa, the capital of Honduras, members of the Nicaraguan Democratic Force said they had received a shipment of Soviet, West German and U.S.-made rifles, machine guns, mortars and antiaircraft guns, Reuter reported.