.S. Ambassador Deane R. Hinton suggested last night that upcoming elections would not provide a political solution to El Salvador's worsening war and there might be no alternative but to seek a military victory over apparently growing ranks of leftist rebels.

At the same time, Hinton expressed doubts in an interview that the United States would give the Salvadoran Army the support necessary to beat the insurgents.

The March 28 vote for a constituent assembly, which would set about writing a constitution with a view toward popular presidential elections within two years, will be "clean, honest elections," he said. "There will be a new government, and the government will no longer be a revolutionary junta imposed after a dictator is thrown out, but will be the legitimate elected government of the people of El Salvador.

"Unfortunately," he said, "that does not end the war. Elections could have been a political solution to the war, and they weren't because those people who allege they're so goddamned popular wouldn't participate and take their chances."

"Those people" are the guerrillas and their political allies who have long maintained that any elections would be fraudulent under military rule. Any leftists who try to run in them, the opposition has said, would be assassinated as others have been before.

The guerrilla leadership has called for a negotiated end to the fighting, with talks among interested parties to begin without preconditions.

Hinton saw only a slight chance of a settlement between the guerrillas and whatever new government emerges from the March 28 elections. Asked, then, where was the way out of the war, Hinton said, "I don't know."

The Reagan administration supports a civilian-military junta, led by Christian Democrat Jose Napoleon Duarte, that contends a communist conspiracy is seeking its overthrow.

"We've never been looking for a military victory," Hinton said. "Now we may be forced to where there's no real choice."

He cited arms supplies that the U.S. and Salvadoran governments charge are being smuggled from communist bloc nations and revolutionary Arab states through Cuba and Nicaragua to the rebels here as "a very critical element" in this possible escalation.

"You can be forced to continue the fight if, as you explore the other options, none of them prove to be workable," Hinton said.

Yet the military option itself has not been conspicuously workable of late and demands resources that, for political or economic reasons, may not be available from the United States.

In December, the Army launched a major offensive in the province of Morazan where large groups of insurgents virtually control the countryside. "That was a big operation," Hinton said. "Over 20 companies. It's the biggest one they ever mounted, and with some of the fewest results, too."

Guerrillas and peasants near the town of Mozote in Morazan told visiting reporters that one thing the Army did do on the sweep was massacre hundreds of civilians.

"From the details I read in the story, there definitely must have been something," Hinton said. "But I don't think it's anywhere near what they say in the propaganda."

Hinton said the embassy had been trying, without success, to confirm the events surrounding the alleged massacre. The embassy has considered sending someone to the scene, but, as Hinton put it, "We're not going to go over to Nicaragua and walk in with the guerrillas. And I don't know whether Mozote is guerrilla territory or not. It sounds like it might be. That's sort of a little problem if it is."

Although Nicaragua does not share a land border with El Salvador, the administration and the Salvadoran junta have charged that weapons are transferred to the guerrillas from Nicaragua into Honduras, and across the border the latter shares with this country.

Asked about the guerrilla attack on the IIopango military air base Wednesday that sources have said crippled the country's small Air Force, virtually destroying five of 14 Huey helicopters on loan from the United States and leaving only three fully operational, Hinton said he could not confirm the figures. "There ain't enough, and now there's fewer than not enough."

Hinton said that the destryed helicopters would probably be replaced by the United States, the decision to do so having been made "in principle," and some more may be added in the course of the year. But even then, Hinton said, there will probalby not be nearly enough to do an effective job.

Hinton would not say just haw many were needed, although 80 is a common estimate. "A hell of a lot more than they've got or are likely to get," he said.

The guerrillas, meanwhile, have shown steadily growing capacity to carry out sophisticated operations, and estimates indicate their strength has nearly doubled in the past 18 months.

Since the failure of an all-out offensive a year ago, however, the insurgents have made numerous offers to negotiate an end to the fighting with the government.

But the stated goal of the Farabundo Marti National Liberation Front is to integrate its own troops with those of the government, and the Salvadoran Army has never found such a proposition acceptable. With its often fierce anticommunist sentiment and deep mistrust of communist political tactics, it has becom convinced that little or no good can come of such talks.

The rebels have steadily moderated the tone of their offers to negotiate over the last year and widened the range of participants to include members of El Salvador's private sector, the Army and other groups.

Hinton said, when asked if he would support a negotiated settlement, "It's too early to answer that. I would go this far: It seems to me that there are some indications from the (Democratic Revolutionary Front) and (Farabundo Marti National Liberation Front) of some change in their attitude."

After the elections, Hinton said, "I would expect the Salvadoran government to reexamine its options."