Downing two warplanes with surface-to-air missiles during a 3-week-old offensive, guerrillas have eroded military morale and prompted Washington to rush about $48 million in aid to the government.

The leftist rebels' successful use of SAMS has also provided fodder for government officials determined to gain restoration of other military aid that was recently slashed by the U.S. Congress.U.S. sources working with the Salvadoran air force, speaking anonymously, said the Salvadoran high command wants Washington to provide more-sophisticated Cobra attack helicopters to replace downed or damaged Vietnam-era "Hueys." The Hueys have been the right-wing government's most significant tactical advantage in the 11-year-old civil war.

On Nov. 23, guerrillas shot down an A-37 attack jet in the eastern province of Usulutan. It was the first time in the war the rebels had brought down a warplane with a missile.

They did it again Dec. 4, downing an AC-47 in the northern province of Chalatenango.

Rebel ground fire brought down a UH-1M helicopter in Usulutan on Nov. 26 and a UH-1H helicopter near the Honduran border on Wednesday. The armed forces said one helicopter gunner was killed in the second incident.

Rebel commander Jorge Melendez, on the clandestine insurgent radio, said U.S. protests over the use of missiles are "cynical and shameless."

He noted it was the United States, in supplying Nicaraguan Contra reb-els with "Redeyes" during the 1980s, that introduced sophisticated surface-to-air missiles to the region.

Rebel spokesman Miguel Saenz acknowledged that the Farabundo Marti National Liberation Front, or FMLN, has acquired Soviet-made SA-7s, SA-14s and Redeyes.

The Salvadoran and U.S. governments contend the missiles were supplied during the past 13 months by Nicaraguan Sandinistas.

Rebel commanders admit privately that some missiles have been provided by their Sandinista allies. But they also say they have bought black market Redeyes from demobilized Contras.

Conversations with air force pilots, infantry officers who depend on air support and U.S. technicians who work with the air force indicate pilot morale is at its lowest point of the war.