Fighting has subsided in the seaport of La Union where three U.S. military advisers were caught during a rebel raid over the weekend, U.S. and Salvadoran sources reported today.

A U.S. Embassy official said the members of the U.S. training team, based at a naval school in La Union, are still on their jobs and were not injured in the fighting.

A spokesman for the Salvadoran Defense Ministry reported that guerrillas of the Farabundo Marti National Liberation Front withdrew from the port late Sunday afternoon. The U.S. Embassy reported that the rebels left a small group of snipers behind to cover the retreat of the guerrilla column.

A sizable contingent of guerrillas, estimated at about 200 well-armed and uniformed men, launched a storng attack on La Union Saturday evening and engaged the Salvadoran military in combat for a full 24 hours.

The attack on La Union, which was the strongest attack on a major town since the January's "general offensive", raised the question of safety for the 41 U.S. military advisers now working here.

At the time of the arrival of the first group of trainers Jan. 17, it was stressed that the military men would not be allowed near "conbat zones." The question of what would happen in the event that an area the advisers were working in became a combat zone apparently never camp up.

Reporters who visited La Union yesterday, after most sniper fire had died down, said evidence showed that the fighting had been intense. Many buildings in the center of the town were completely riddled with bullet holes, and the streets were strewn with spent ammunition.The reporters speculated that the guerrillas had staged a lightning attack with no intention of holding the town or returning to it in the near future, and that they had suffered few casualties. Only two corpses were lying in the street.The Army reported no casualties among its troops.

U.S. Embassy sources said what happened in La Union was a relatively minor event.

"We at no point felt our trainers were in any danger," said an embassy official. "We did not consider it necessary to pull them out."

Asked if the restriction imposed on the advisers of staying away from combat zones did not imply that they should make every effort to get away from a combat zone once fighting breaks out, a U.S. official replied: "That depends on what you mean by a combat zone. La Union has previously been considered a safe area, and the weekend's fighting is an isolated incident. Countinued fighting would prompt a reevaluation of that."

The number of U.S. military trainers here has decreased since March when U.S. sources said a total of 56 were to be stationed here. One six-man team stationed at La Union was pulled out and replaced by the three-man team that is there now.

The embassy said the number would decrease again next month when a 14-member team of helicopter maintenance instructors is to be sent home.