SAN SALVADOR, JAN. 2 -- Leftist guerrillas shot down a U.S. military helicopter in the eastern part on the country today, killing three American crewmen on board, the rebels said.

A radio rebel broadcast said a guerrilla unit brought down the Huey helicopter near the village of Lolotique, 75 miles east of the capital, but did not say what weapons were used.

{Reuter quoted an armed forces spokesman as saying the helicopter was downed with a ground-to-air missile, instantly killing all three aboard. However, a U.S. Air Force source said two of the crew died of bullet wounds and the third of injuries on impact. The same source said one of the three dead Americans was an Air Force lieutenant colonel.}

The bodies of three crew members were found by a guerrilla column that searched the wreckage and they found bodies carrying U.S. military identification, said the broadcast by the Farabundo Marti National Liberation Front rebel organization, known as FMLN.

It identified the pilot only as David Scott, the copilot only as Pickett and the engineer only as Dawson.

The U.S. Embassy refused to confirm or deny the report other than say a U.S. helicopter was missing. "We have heard the FMLN report and the only thing we can confirm is that one of our helicopters is missing," embassy spokesman Jeff Brown said. In Washington, spokeswoman Maj. Kathy Wood said only that the Pentagon had reports of a U.S. military helicopter overdue on a trip to Honduras from El Salvador.

U.S. military advisers, limited by Congress to 55, often use helicopters in moving among Salvadoran army units. Seven Americans on official duty here, including five military personnel, had been killed until now in the 11-year civil war.

A military source, who insisted on anonymity, said several Salvadoran army infantry units were sent to search for the wreckage and retrieve possible victims.

The broadcast by the guerrilla Radio Venceremos said rebels also found military two-way radios, a Browning pistol and an M-60 machine gun.

Rebel forces in the area did not know it was a U.S. helicopter, the broadcast said. It added that "at the time it was a war craft that was overflying a war zone . . . and within this context it was shot down."

The helicopter was brought down by antiaircraft fire at about 2:30 p.m. EST, the rebel broadcast said but did not specify if artillery or surface-to-air missiles were used.

On Tuesday, the Sandinista army in Nicaragua said some of its officers had secretly sold surface-to-air missiles to the Salvadoran guerrillas. The guerrillas used the missiles for the first time in its November-December offensive to shoot down two government aircraft.

Last Friday, the Farabundo Marti National Liberation Front broadcast that it was ending a six-week military offensive and was ready for new peace talks. It said a cease-fire was to take effect Dec. 31.

The rebel offensive was closely watched because of congressional action last year that halved U.S. military aid to El Salvador, but with a provision that the cut would be restored in the event of a major rebel offensive. The FMLN said its drive, begun Nov. 20, caused 1,190 army casualties.