U.S. and Honduran officials and Nicaraguan rebels denied yesterday any connection with an alleged helicopter raid on a Nicaraguan military observation post that the Sandinista government says it repelled with antiaircraft fire.

Nicaragua's leftist Sandinista government said that three helicopters crossed the border from Honduras Monday morning and that two of them crashed in Honduran territory after they were hit.

A U.S. Embassy official in Tegucigalpa, the Honduran capital, said that the Nicaraguan report might be part of a "disinformation" campaign and that it was untrue.

"There is nothing to substantiate this story, according to the Hondurans and according to our sources," the official said.

In Washington, a White House official said that he did not think there was anything to the Nicaraguan report or that any helicopters had been shot down.

A Honduran military spokesman described as "totally false" Nicaragua's charge that the helicopters had entered Nicaraguan territory from Honduras, news agencies reported. The government declined to comment on whether helicopters had crashed on its territory.

Nicaraguan officials said yesterday that photographers were being flown to the border, where they said the wreckage of the helicopters could be seen. The Nicaraguan Embassy in Washington said the photographers' airplane had encountered mechanical difficulties and had not returned from the border yet.

Bosco Matamoros, spokesman in Washington for the rebel Nicaraguan Democratic Force, said the group had no helicopters and did not conduct aerial operations. He said the reports that two helicopters were hit were "a typical Sandinista maneuver intended to alarm Congress and North American public opinion."

President Reagan has been campaigning hard for aid to the rebels, which Congress is debating.

The reports of the helicopter incident come at a time of increased tension on Nicaragua's borders with Honduras and Costa Rica. The Nicaraguan Army has been conducting a major offensive along both borders against the rebels.

Costa Rica put its only military force, the Civil Guard, on alert late Wednesday night following an incident six days ago in which two Civil Guardsmen were killed and six wounded during what Costa Rica says was an incursion by Nicaraguan troops.

Yesterday, Costa Rican Foreign Minister Carlos Jose Gutierrez left for Washington, where he was to call on the Organization of American States to investigate the border clash, Washington Post correspondent Robert J. McCartney reported from San Jose, Costa Rica.

Nicaragua has denied that its forces crossed into Costa Rica and yesterday called the Costa Rican move in the OAS a U.S. ploy to bypass efforts by the Contadora group to negotiate peace in Central America.

"There is no doubt that the United States is behind this matter, to deliver a political blow to Contadora," Nicaraguan Deputy Foreign Minister Victor Hugo Tinoco said, The Associated Press reported from Managua.

"All this is part of [the U.S.] campaign to pave the way toward a direct intervention of North American troops in Nicaragua."

Costa Rica has downgraded its diplomatic relations with Nicaragua as a result of the border incident.