Members of the U.S. military mission helping El Salvador fight its war against leftist rebels now advocate increasing the number of U.S. military advisers here above the current ceiling of 55 men, an informed Western source has said.

The military source, who is knowledgeable about both Salvadoran military operations and the thinking of the U.S. military mission here, yesterday praised the fighting performance of three U.S.-trained battalions in two recent campaigns but criticized the tactics of regular Army units, which in a battle in early June led to the rout by guerrillas of two Army companies with about 300 men.

Asked for comment, a U.S. diplomat here said today that the embassy was "not giving any serious consideration to raising the number of trainers." Approximately 30 advisers are in El Salvador at the moment, down from a high of 55 last year, and the average number this year has hovered around 40, embassy officials said.

The increase in the number of advisers is being suggested to allow teams of advisers to be based at headquarters of Salvadoran Army brigades around the country, the military source said. Most of the advisers currently are based in San Salvador and go out by day to train Salvadoran troops in nearby camps.

The source said that the Salvadoran high command had been approached about introducing teams at the brigade level and that it supported the idea. Although Salvadoran military officials today were unavailable for comment, the government in the past has said that, while it welcomes any increase in U.S. aid, it does not need additional U.S. personnel to win the war.

The military source, who spoke on the condition that he not be identified, said he saw no quick end to the war, as fighting here in recent weeks has reached its heaviest levels since the March 28 elections. The United States, he said, is "in for the long haul. Gringo impatience is not going to solve it." He described a scenario for winning the war utilizing U.S.-promoted tactics monitored by U.S. advisers.

The source said that a U.S. aid cutoff--which has been debated in some congressional committees--could result in a rightist takeover of the present pro-U.S. high command and that the Salvadoran armed forces, "tightening their belts," could hold out for only about a year without U.S. backing.

He said the U.S. military team was attempting to instill more effective counterinsurgency tactics in regular armed forces units, including the use of "saturation patrols" by small units operating at night as well as by day to "deny the night to the guerrillas."

To do that, he said, the team would like the U.S. trainers to be formed into "mobile training teams" located in the headquarters of the six brigade zones into which the military has divided El Salvador. The informal U.S. ceiling of 55 total trainers should be increased in order to allow training teams to work in the headquarters around the country, he said, calling the brigades the "weakest link" in the influence of the U.S. military team over Salvadoran performance in the field.

The brigade commander in the provincial capital of Usulutan recently was removed, the source said, because of his reluctance to utilize small-unit tactics.

The military source did not say how many additional trainers would be needed to expand the U.S. military operations into the brigade headquarters. When proposed in the past for use in Central America, such teams generally have been said to have 12 men each.

The U.S. diplomat said the idea of such military teams was "worth exploring" but that "we haven't really studied that at all."

Fighting in the civil war has intensified following a lull in April and May that came after a largely unsuccessful guerrilla offensive around the time of the March 28 elections. In a flurry of activity here, apparently in anticipation of the congressional requirement that the Reagan administration certify progress in its Salvadoran policy before the end of July, the embassy for the first time has made available regular briefings on the military situation.

In the first test of a Salvadoran battalion trained for four months at Ft. Bragg, N.C., and of two other U.S.-trained units here, the Salvadoran Army launched at the end of May a so-called "clean-up" offensive involving about 4,000 troops in the northwestern guerrilla stronghold of Chalatenango. The Army claimed it killed 135 "subversives" before calling off the 10-day sweep.

On May 5, however, the Farabundo Marti Liberation Front forces overran the towns of Perquin and San Fernando in northern Morazan Province.

The Army's first attempt to retake the town was a failure, the military source said. The guerrillas were prepared for a prolonged stay, with ample ammunition and a force estimated as high as 800 men, he said.

Two Army companies were sent north to Perquin on June 9 and ran into an ambush four miles outside of town. "Coordination was abysmal," the source said. "They walked out from under their artillery support and got the s--- kicked out of them."

A few days after the ambush, the army called in three U.S.-trained battalions: Atlacatl, which was trained in 1981; Belloso, trained in Ft. Bragg; and Atonal, trained this year in El Salvador. The crack battalions performed well, the source said, especially the Belloso.

But the key to the government counteroffensive, the source said, was the arrival in El Salvador on June 15 of six U.S.-supplied A37 fighter-bombers. They broke up the roadblocks and on June 22, after a 13-day occupation, the guerrillas withdrew from the towns. Nevertheless, the military source said that "initially at least" the fighting was a psychological victory for the rebels.