President Carter has decided to resume U.S. military assistance to El Salvador, freeing $5 million in "non-lethal" equipment for the Salvadoran armed forces, along with at least two of six requested transport helicopters on lease and the services of several U.S. military technicians, informed sources said yesterday.

Also programmed in the forthcoming aid package, officially suspended last month following charges that Salvadoran security forces were involved in the murder of three American nuns and a lay worker, is a separate, previously approved plan to send one six-member Military Training Team from the U.S. Army to advise Salvadoran Army Headquarters in counterinsurgency tactics.

The decision to resume aid, sources said, was made yesterday by Carter with the concurrence of Secretary of State Edmund Muskie, who earlier had expressed reservations on grounds that the Salvadoran government could not adequately control its own forces.

Both Muskie and the U.S. Embassy in El Salvador, which had similar reservations, now believe that leftist guerrillas, who last weekend announced the start of a "final offensive" in their drive to overthrow the government, have received substantial new arms shipments and pose a strong threat to the U.S.-backed regime.

According to the sources, members of Congress will be briefed on the decision this morning and it will be announced at the State Department briefing at noon. They said the "material has already been readied" in preparation for the decision and that the helicopters, along with at least three U.S. pilot trainers and technicians, could be on their way to El Salvador as early as today.

U.S. military aid to El Salvador has been the subject of debate within the Carter administration almost since the day in October 1979 when a group of young Salvadoran Army officers staged a coup against the last in an unbroken series of rightist generals who had ruled the country, along with a small landowning elite, since the early 1930s.

Following on the heels of the July 1979 Sandinista overthrow of Anastasio Somoza in nearby Nicaragua, the Salvadoran coup, and the installation of a reform-minded coalition government was viewed by the United States as the route to avoiding a Nicaraguan-style civil war.

The State Department quickly reprogrammed $5.7 million in 1980 funds for military assistance to bolster the new government against increasingly active guerrilla forces who had gotten their own impetus from the Nicaraguan revolution.

But because of charges that some members of the Salvadoran security forces -- consisting of a wide range of authorities including the Army, National Guard, national police and treasury police -- still were engaging in the kinds of repression against suspected peasant "subversives" common under the previous government, the aid was restricted to "nonlethal" items such as transportation and communication equipment.

For fiscal 1981, the administration budgeted $5 million in similar equipment, including jeeps, trucks, military uniforms and $2.7 million in loan funds for maintenance and refurbishing of six Bell helicopters that were to be "leased" to the Salvadoran government on a no-cost basis.

At the same time, various administration programs were proposed, then modified or discarded, to send as many as three 12-man U.S. Army military training teams to the Salvadoran countryside to advise the armed forces in counterinsurgency techniques. Most of those plans were opposed by Muskie, the State Department human rights bureau and U.S. Ambassador to El Salvador Robert White on grounds they would demonstrate too much on-the-ground support for the shaky government.

Last month, a compromise proposal between Muskie and White, on the one hand, and the Defense Department and the National Security Council, on the other, was approved calling for one six-member military training team, to be stationed at Army headquarters in San Salvador.

All of those programs were held in abeyance, however, first because of congressional inquiries, and eventually because of the murder of the four American women.