Salvadoran army attacks against the leftist rebel group that claimed responsibility for the June slayings of four Marines and nine other persons in San Salvador were mounted as part of the military's normal antiguerrilla operations, and not at the request of the United States, a Pentagon spokesman said yesterday.

But Defense Secretary Caspar W. Weinberger, while acknowledging that there was no indication that the trigger men in the June 19 assault had been killed or captured, insisted that Salvadoran troops had exacted a measure of retaliation in their recent attacks against the Central American Revolutionary Workers' Party (PRTC).

"I think the effectiveness of this group has been weakened, without question," Weinberger told the Associated Press.

Wednesday, Weinberger told the Mutual Radio Network that Salvadoran troops, with U.S. "assistance," had killed or taken prisoner "a number of the people who had participated in that killing" at a sidewalk cafe in the Salvadoran capital.

Pentagon spokesmen moved quickly to clarify the secretary's remarks, saying he did not mean to suggest that the actual gunmen had been caught or killed. The spokesmen said Salvadoran troops, guided by U.S. intelligence, attacked PRTC positions, killing 21 guerrillas, wounding 40 and capturing nine, including two key leaders.

Salvadoran military officers confirmed the attacks against the PRTC, the smallest of the rebel forces that have battled the Salvadoran government for five years, but described the attacks as routine and played down the importance of U.S. intelligence.

At a Pentagon briefing yesterday, Fred S. Hoffman said the recent Salvadoran campaign against the PRTC was part of a "continuing antiguerrilla, antiterrorist operation" by special Salvadoran troops trained and armed by the United States. Asked if the campaign was mounted at U.S. request, the spokesman replied, "No. The military forces of the government of El Salvador plan and execute their own operations."

In his interview with the AP, Weinberger said, "We helped them [El Salvador] with intelligence. And they went in and, in the course of a sweep operation, managed to be very effective in killing and capturing members of this group.

"And this was the group -- they're satisfied and we're satisfied -- which had killed the Marines in San Salvador," Weinberger said. "And this was an effective way, one effective way of dealing with terrorists."

Asked if U.S. officials believed that some of the guerrillas who planned the cafe attack had been captured or killed, he said:

"The Salvadorans indicated that some of the people captured or killed were leaders of this group. It's a small group, so I don't know if anyone wears a uniform labeled 'leader' or not."

About two weeks ago, the Reagan administration, which has been under pressure to show resolve against terrorists in the wake of the cafe attack and the Beirut hostage crisis, offered a reward of as much as $100,000 for information leading to the capture and prosecution of the Marines' killers.

A State Department spokesman said yesterday that the reward was still available.

"Our commitment to find and bring to justice those who carried out the murder continues," the spokesman said. "The Salvadoran government shares that commitment. In any event, rewards are not paid to dead suspects."

At his briefing, Hoffman also said that the nuclear-powered aircraft carrier USS Eisenhower and its battle group were conducting "routine operations" off the east coast of Central America. He was responding to reports from Nicaragua that officials there were alarmed because the Eisenhower had been sighted off that country's coast.