The Salvadoran Army is beginning to show signs of strain in the face of the three-day-old leftist offensive here.

Of the military's 10 French-made Alouette and Lama helicopters, only two are still flying. At least one was shot down by guerrillas near the Guatemalan border last week, according to official sources, another was damaged over the province of Morazan and the rest are suffering from lack of spare parts and maintenance as they have been in continuous use.

The guerrillas claim to have downed three.

At the same time the leftists are using ever more sophisticated weapons in greater quantities.

Even the U.S. ambassador to El Salvador, Robert White, who has consistently opposed "uping the ante" here by renewing and increasing U.S. military aid -- especially since its suspension last month amid allegations that Salvadoran soldiers were involved in the murder of four American women -- said today that such aid may at this point be necessary.

"I am very concerned about the obvious increase in supplies to the guerrillas from the outside which seems to threaten escalation on the part of the extreme left," White said.

[In Washington, informed sources said Tuesday night that President Carter has decided to resume U.S. military assistance to El Salvador and that the announcement will be made Wednesday.]

Despite the claims of the Salvadoran government that the guerrilla push has been controlled, sporadic fighting continues nationwide. The towns of San Francisco Gotera to the northeast and Zacatecoluca to the south were the scenes of major confrontations this morning, according to military spokesmen, and there was brief fighting in the neighborhoods of San Salvador again today.

There is still no substantial evidence, however, that the people of El Salvador are willing to rise up en masse behind the guerrillas, estimated to include 5,000 armed and trained rebels plus several thousand more ragtag militia. A general strike called by the Revolutionary Democratic Front last night has thus far garnered only half-hearted support, at least in the capital, where most shops and factories were open and most buses running by early afternoon.

Salvadoran government sources have repeatedly blamed Nicaragua and Cuba for much of the current surge in shipments of material to the revolutionaries here, although no solid proof has yet been offered.

Fears remain in the U.S. Embassy that a renewal of U.S. military aid will seem to endorse some of the brutality shown by the Salvadoran armed forces, which total about 15,000 troops. But as the fighting becomes more open and extensive here, that has become something less of a preoccupation.

More worrisome to some officials is the possibility that supplying sophisticated equipment that requires the presence of U.S. personnel for training and maintenance will lead to increasing and increasingly direct U.S. involvement in the conflict.

"It could shift the whole emphasis of the thing," one diplomat said recently. "They [the Salvadoran military] have been doing their own jobs up to now. But this could make them more and more dependent on U.S. technical advisers. That's what happened in Southeast Asia."

The situation here is still very uncertain, especially in the countryside. The offensive is only 72 hours old and the guerrillas under the command of the Farabundo Marti National Liberation Front, still using hit-and-run tactics, are reported to be holding many of their forces in reserve. It is not clear why.

But the cost to El Salvador already has been high. Reliable figures are not available but unofficial estimates of casualties over the weekend, including many civilians, run high into the hundreds. That is in addition to an estimated 10,000 people who died at the hands of extremist leftist and rightist terrorists and the armed forces last year before the real civil war actually began.

Yesterday three journalists were injured by what was apparently a guerrilla-laid mine in a remote rural area to the north of the capital and this morning one of them, South African Ian Mates, 26, died from head wounds. He was a cameraman for UPITN, the newsfilm service of United Press International and Independent Television News in London. Two American photographers who were with him but less seriously injured, John Hoagland, 33, and Susan Meiselas, 32, were flown to Miami today for further treatment and observation.

A free-lance journalist from Venezuela, Nelson Arietta, reportedly was arrested at his hotel by national police yesterday for alleged links with the guerrillas. One report said he would face charges of "being a spokesman for the terrorists."