Guerrilla forces in El Salvador are reported to have executed 42 government soldiers taken prisoner last week and to have mutilated some of the bodies, a move that may signal a major change in rebel policy, the State Department said yesterday.

Spokesman Alan Romberg released the partial text of a declassified cable from the U.S. Embassy in San Salvador. It said the "guerrilla atrocity" occurred early on May 25, 2 1/2 weeks after guerrillas executed 16 civil defense fighters who surrendered in the town of Cinquera.

Previously, guerrillas have sought to undermine soldiers' will to fight by treating any who surrender with kindness, taking their arms and returning only propaganda, asking them not to fight for the government again and turning them over to the Red Cross.

"It is too early to tell if a trend has been established, but it appears that the guerrillas may have decided to up the ante," the cable said.

"Why remains a mystery for the moment, as their tactic of capturing, stripping and quickly releasing soldiers unharmed had been so successful."

The second incident occurred when an 82-man force of El Salvador's 5th Brigade ran out of ammunition after an eight-hour battle with guerrillas trying to take a bridge near San Francisco Chamoco in the province of San Vicente, northeast of the capital, the cable said, quoting "a source."

"The guerrillas demanded the soldiers surrender and they did," the cable said. "Following the surrender a number of soldiers were placed face down on the ground and executed by the guerrillas." It added that the source had reported that 70 to 80 percent of the 42 bodies "have head wounds and powder burns and have suffered additional mutilations (ears cut off, arms severed)."

Five wounded soldiers returned to headquarters and 36 were on their way back, the cable said, while guerrilla Radio Venceremos reported the capture of one lieutenant and four soldiers.

There was no explanation of the discrepancy in the numbers.

In the earlier incident, which occurred on May 8, the guerrillas shot 16 men who had unsuccessfully defended Cinquera, in Cabanas province, after a 12-hour fight. The civilians' thumbs were tied behind their backs before they were shot.

Romberg also said that he could not confirm claims from Salvadoran leftist groups that the Popular Liberation Front was responsible for the first American military death in the Salvadoran war last week, Lt. Cmdr. Albert Schaufelberger. He was shot in his car last Wednesday evening, the day of the incident near San Francisco Chamoco.

The State Department also announced that it has decided against recommending that Salvadoran refugees in the United States be granted "extended voluntary departure" status, which would allow them to remain in this country indefinitely. Such a move "would encourage further illegal immigration to the United States from El Salvador, a country from which there has long been substantial illegal immigration," a department statement said.

"Moreover, Salvadorans in the United States pass through other countries, before they arrive here, in which they could obtain refuge."