A Nicaraguan patrol boat opened fire yesterday on a U.S. Navy helicopter in international waters near the Nicaraguan coast, the State Department annnounced.

The brief announcement said the helicopter from the USS Trippe, which was engaged in unspecified "routine naval activities," was the target of heavy machine-gun fire shortly after 3 p.m. (EDT).

The U.S. statement said the helicopter was unarmed, and that it was not damaged in the incident. The Trippe did not return the fire, the statement said.

Both the helicopter and its mother ship, a frigate, were "in international waters more than 12 miles from the coast" at the time of the incident, according to the State Department.

Nicaragua, like many Latin American nations, claims territorial waters of 200 miles, a claim the United States does not accept.

No details were immediately available on what the U.S. frigate was doing in the area, which was unofficially identified as the Gulf of Fonseca, the body of water between Nicaragua and El Salvador that has been under suspicion as a channel for arms supplies to Salvadoran rebels.

U.S. officials said no special maneuvers are under way in that area. The aircraft was reported to be an SH2F helicopter of a type often used for anti-submarine patrols.

Shortly after the incident was announced, Nicaraguan Ambassador Francisco Fiallos was called to the State Department to receive an official protest.

U.S. intelligence-gathering ships have operated off the Nicaraguan coast since late last year, prompting protests from Managua.

In February it was reported that the USS Caron, a destroyer outfitted with specialized electronic gear, had been dispatched to the Gulf of Fonseca to replace the USS Deyo, a similar ship assigned there for two months to monitor the flow of arms through the area to El Salvador.

Late in March, several clashes were reported between Nicaraguan, Honduran and Salvadoran craft in the waters off the heavily contested area of Central America. In one incident, Honduran fighter planes and a Nicaraguan patrol boat clashed in the Caribbean, with each side claiming the other had fired first.

On April 15, Nicaragua protested the presence of the destroyer USS Koontz, apparently a surveillance ship, off its Caribbean coast. A Nicaraguan note charged that the U.S. craft was preparing for military intervention in the area and that its presence was "a provocation."

The most recent incident came at a time of continuing tension between the United States and Nicaragua.

Two months ago today the United States presented Nicaragua with an eight-point plan for an accord between the two nations, the follow-up to proposals from Mexican President Jose Lopez Portillo for a diplomatic settlement in Central America.

Nicaragua quickly said it was willing to discuss the U.S. plan and proposed discussion points of its own as well, and in the days to come Managua persistently asked for the start of high-level negotiations on the issues.

The Reagan administration, however, so far has declined to set a date for the beginning of the talks. Nicaragua's deepening internal troubles and a recent visit to Moscow by Sandinista leader Daniel Ortega have been cited by some in Washington as reasons to go slow in any move toward possible rapprochement.