The State Department's Latin America bureau, despite the objections of its embassy in the field and the known reluctance of Secretary of State Edmund Muskie, has recommended presidential approval of a resumption of military aid to El Salvador that would place six U.S. helicopters, several million dollars' worth of equipment and at least six U.S. military advisers in that war-torn country as early as next week, informed sources said.

The sources said that the resumption of aid, which was suspended Dec. 5 following reports that Salvadoran security forces were involved in the slaying of three American nuns and a lay worker there, is also strongly supported by the National Security Council and the Defense Department.

Muskie, who has vetoed increased assistance to El Salvador's military-civilian coalition on several occasions, met today on the issue with presidential national security adviser Zbigniew Brzezinski and Defense Secretry Harold Brown. Sources said representatives of the three men have scheduled another meeting for Monday, after which they plan to present their recommendations to President Carter, who is expected to make a decision early next week.

Defense and NSC officials, as well as the State Department bureau headed by Assistant Secretary of State William G. Bowdler, have advocated resumption of military aid on grounds that a promised investigation of the slayings is under way with FBI assistance. They have argued that the Salvadoran government has taken steps to consolidate moderate and civilian control over extreme rightist members of the security forces believed involved in widespread political violence and that the government should be encouraged.

The aid in question, from the fiscal 1981 appropriation, includes $2.3 million in so-called nonlethal military transportation and communications equipment, as well as $2.7 million for the upkeep of six Bell helicopter transports.

The helicopters are to be offered to el Salvador under a "no-cost lease" agreement tantamount to a loan of the equipment that does not require payment or congressional approval. Ten helicopters, which theoretically are to be returned to the United States should a one-year lease not be renewed, already have been sent to Honuras under a similar arrangement.

In addition to several U.S. military officials to instruct in the upkeep and use of the helicopters, the program also reportedly includes one or more training teams totaling from six to 12 U.S. soldiers to instruct and advise the Salvadoran security forces headquarters in San Salvador on counter-insurgency techniques.

In a series of interdepartmental meetings and congressional briefings, those in favor of the aid have maintained that U.S. intelligence reports show marked increases in both the quality and quantity of weapons flowing to El Salvador's leftist guerrillas recently from sources that included Nicaragua, Cuba, Vietnam, the Palestine Liberation Organization, Libya and Eastern Europe.

Arguing that there is no time for delay -- and noting that the request for military aid has come from civilians in the Salvadoran government rather than the military -- they have pointed to the guerrillas' own statements promising a "final offensive" in El Salvador before the Jan. 20 inauguration of U.S. President-elect Ronald Reagan.

In reports yesterday from El Salvador, news agencies said that the armed forces have been placed on high alert but that the anticipated all-out offensive had failed to materialize. There were two reports of clashes around the capital of San Salvador, with officials reporting 20 guerrillas killed. Seven civilian patrolmen were said to have been assassinated, apparently by leftist guerrillas.

Informed sources say that, while Muskie has not yet made a firm decision on what he will recommend to Carter, both he and U.S. Ambassador to El Salvador Robert White believe that progress in the slaying investigations has been sufficient to justify the aid resumption.

At the same time, the sources said, while they agree that the guerrilla armory has been substantially increased over the past several months, both men are skeptical of intelligence reports that they believe have been repeatedly inflated in the past to justify the desire of U.S. defense and security officials to intervene actively in the Salvadoran situation. Both Muskie and White also are said to believe that the Salvadoran government should be given time to prove that the restructuring it has announced is both effective and permanent.

The current battle over El Salvador within the Carter administration has somewhat altered the traditional lineup over this issue and others that repeatedly has found the State Department, and particularly the Latin American bureau, as a whole opposed to Brzezinski's NSC and the Pentagon. a

Although some State Department officials here said they believe the Reagan administration will both resume and increase the level of U.S. military aid to El Salvador, and argued that the Carter administration should go out of office upholding human rights as a primary policy concern until the last, a report from the Latin American bureau now on Muskie's desk concludes the aid should be resumed.

Some department observers suggested somewhat facetiously that former upholders of human-rights restrictions now are more interested in currying favor with the incoming administration than sticking to Carter's goals.

The chronology of recent U.S. assistance to El Salvador began in October 1979, when a group of so-called progressive military officers overthrew a rightist military government and formed a coalition junta with left-of-center groups. The U.S.-backed junta pledged widespread reforms that would benefit the vast majority poor in the small Central American country of nearly 6 million people.

Another of its pledges was to stop widespread repression and human rights abuses by the armed forces, while still combatting a growing threat from leftist guerrillas determined to seize power.

In January last year, civilians in the coalition resigned en masse, charging that military repression had not ceased, and forming a political coalition with the guerrillas. Another government formed with members of the centrist Christian Democratic Party fell apart for the same reason in March.

A third coalition was then formed with two other Christian Democrats, Antonio Morales Erlich and Jose Napleon Duarte. They declared a massive land-reform program in effect and, according to U.S. officials at the time, made large strides toward keeping the military in line.

Beginning last summer, however, the level of political violence increased. Nearly 10,000 political murders were counted during 1980, the majority of them attributed by church officials who did the counting to extreme rightists believed closely tied to conservative elements within the military. Although the Carter administration budgeted $5 million in military aid for the junta during fiscal 1981, its dispersal was delayed, both by executive decision and congressional advice, pending more progress in moderating government forces to prevent human-rights abuses.

Crisis came at the end of November, with the murder, blamed on the right, of six leftist political leaders in San Salvador. That incident was closely followed by the killing of the nuns and the suspension of all U.S. military aid. A U.S. presidential commission visited San Salvador and came back with reports that an investigation was being mounted, and the junta had promised more restructuring. iEventually economic assistance was resumed.

The restructuring, proponents of the military aid resumption believe, has now come a long way with the installation of Duarte as a civilian president and the removal last week of at least one of the most suspect rightist military officers.

Some of the most outspoken opposition to the resumption plan so far, sources said, has come from Patricia Derian, head of the department's human-rights bureau. Over the past month, since the aid was first suspended, Derian's office has been involved in a largely losing battle of memos as various items of assistance have been gradually reinstated.

Although the public State Department announcement of aid suspension implied all forms of military assistance were to be withheld, subsequent decisions have allowed the resumption of U.S. training of members of the Salvadoran armed forces in Panama, and the continuance of aid in the 1980 pipeline.

The human-rights argument is based on the fact that, despite the urgency of the situation, and the administration's Dec. 5 promise that aid would be withheld "pending clarification of the circumstances of the killings," there is no reason to believe the investigation thus far has been either rapid, comprehensive or conclusive.

Two FBI officers currently are providing technical assistance to the Salvadoran commission investigating circumstantial evidence that government security forces were involved in the shooting deaths of the American women, who were shot -- and two of them raped -- while traveling to the capital from the airport.

According to State Department sources, the FBI submitted an "interim" report on the investigation to Carter several days ago. It noted that the commission had interviewed Salvadoran soldiers on duty the night of the slayings and was taking depositions from other possible witnesses. But, the sources said, the investigation had been "held up" for nearly three weeks by the Christmas holidays.

The human rights bureau also has argued that a promised decrease in the level of violence, allegedly perpetrated by right-wing paramilitary extremists allied with government forces, has not occurred. It has pointed to the slayings last Saturday in a San Salvador hotel of two other Americans, agrarian-reform advisers Michael Hammer and Mark Pearlman, along with the head of the Salvadoran land reform institute.

Although some U.S. officials have expressed a belief that those slayings were also to be blamed on the right, there has been no conclusive evidence presented thus far to implicate either the right or the left.