In the four days since the assassination of El Salvador's archbishop Oscar A. Romero, U.S. diplomats here have expressed reservation over the merits of U.S. military aid now being considered in Washington for El Salvador's ruling civilian-military junta.

While they do not explicitly oppose the aid, which administration officials earlier this week told Congress is essential to restore peace here, the diplomats have repeatedly charged Salvadoran military factions with "excesses" of violence.

Previously, the U.S. diplomats had blamed much of the country's killing on rightist paramilitary groups operating independently of the armed forces, and portrayed the military as the only force capable of controlling the extreme right and left.

Speaking to a mostly conservative group of businessmen here today, U.S. Ambassador Robert E. White called these military elements "the enemy within."

"There rests a heavy responsibility, in the view of the United States," White told the businessmen, "on the officer corps of the armed forces to put an end to the abuses . . . If the abuses continue, you are going to find, just as in Nicaragua, a countryside radicalized by barbaric acts."

According to figures compiled by the Roman Catholic church here, 689 people have been killed since the beginning of the year in political violence. The church lists the majority of them as peasants killed by the armed forces. The army maintains that those it had killed have died in military confrontations with guerrilla groups.

The essential question in the minds of several embassy officials here is whether the proposed military aid package, consisting of $5.7 million in transportation and communications equipment, will help bring the oppressive elements of the armed forces under control, or encourage them.

The original aid package, drawn up several months ago when the United States decided on strong support for the coalition junta, called for sending U.S. army trainers and weapons. That program was reworked when it became clear that neither the U.S. nor the Salvadoran governments was prepared to accept the political costs of appearing to contribute to military repression.

The $5.7 million would come out of reprogrammed fiscal 1980 funds, and therefore does not need congressional authorization. The administration, however, is required to notify Congress of its plans and members of both the House and Senate have raised questions over the advisability of the aid. Should any congressional committee raise serious objections, it is considered unlikely that the administration should implement the program.

"We are in a tough situation," White told reporters earlier this week. "Our success will depend, to a certain extent, on our willingness to provide [the military] with material. If you stop the supplies, that diminishes your influence and eventually it evaporates.

"On the other hand, there is the problem that we do not want to give the impression that we totally support the armed forces as presently constituted. In some quarters, that would be interpreted as U.S. approval of the excesses. I assure you, that is not true."

White also said that he believed the extreme right, possibly using a hired professional from outside the country, was responsible for murdering Archbishop Romero.

In addition, White declared that the nation's most popular leftist leader, Juan Chacon of the Popular Revolutionary Bloc, had been assassinated today. But Chacon later held a news conference and said the death report was part of a plot "backed by U.S. imperialism." If he had been killed, the impact could have been intensely violent.

Before Romero's assassination Monday night, U.S. diplomats and El Salvador's fragile military-civilian government were showing first tentative signs of optimism about a possible peaceful solution to El Salvador's conflicts.

Violence by marxist extremists and right-wing forces both inside and outside the Army continued, but the massive agrarian and economic reforms initiated at the beginning of the month seemed seriously to have undercut the left by preempting some of its most potent promises to the impropoverished peasant majority here.

The extreme right also appeared more isolated as moderate conservatives began to accept the reality of the reforms, if not to endorse them.

"Understanding was growing every day," said one member of the government junta. Both members of the government and the U.S. Embassy were making a concerted effort to reach an understanding with the archbishop, who had been one of their most vocal and powerful critics. Ambassador White attended the archbishop's last Sunday mass.

The murder of Romero is generally seen as an effort to destroy such peaceful initiatives and reduce the chances of nonviolent solutions. It was widely expected to precipitate rioting or popular insurrection in the way that the murder of Martin Luther King Jr. did in the United States a dozen years ago or the murder of newspaper publisher Pedro Joaquin Chamorro did in Nicaragua in 1978.

That it has not -- at least not yet -- is due partly to the shock felt by the people of El Salvador, apparent on the faces of the thousands of mourners who are passing by the glass-topped coffin of the archbishop lying in San Salvador's vast, unfinished cathedral.

As one church worker put it, "This was the final blow. The people here are sick of the killing. The murder of a priest during a mass -- the murder of the archbishop -- it is unbelievable. They want the killing to stop."

Few people actually expect that is will. Militant leftists have called for a general strike to be signaled by a general power blackout at any moment. A similar attempt at a strike early last week did not receive great popular support but ended with rioting in the streets and the deaths of more than 50 people by official counts. Other estimates range upwards of 100.

"The clear danger to the government is that the extreme left will try to take possession of the archbishop's popularity and moral authority, now magnified by his violent death," said White.

But there is also the possibility, expressed as a fear by many leftists, that the extreme right is trying to provoke them into confrontations before they are ready, or sufficiently armed, in which case the right could crush them altogether.

Leftist Chacon publicly called yesterday for "serenity" in the aftermath of Romero's death.

Hundreds of thousands of mourners are expected to come to San Salvador for Romero's funeral on Sunday. Passions and fears are expected to be high. In this hate-filled country any such gathering presents tremendous risks of violence.

Some diplomats and government officials say they believe that if the current regime can weather this crisis it will emerge stronger. But the problems that had this country hovering on the briks of full-scale war for more than a year are likely to remain for some time.

Deep and destructive divisions run through virtually every institution in the nation. Voices of conciliation have been eliminated systematically through murders or threats.

The Catholic church is so deeply split that four of the country's five bishops -- all conservatives -- were specifically told by priests close to the archbishop not to come to Romero's first memorial service on Tuesday.

The leftist clergy who had supported Romero now openly express worries that the Vatican, which was often troubled by his outspoken opposition to the government here, will appoint a conservative as his successor.