U.S. Ambassador Deane R. Hinton spoke out for the first time tonight against "serious excesses" by the government of El Salvador in its war against guerrillas and warned that Congress will refuse further aid if the human rights situation here does not improve.

American citizens understand "some abuses by those engaged in battle," Hinton said in a speech to a business group, "but there is a limit, and at times this government has treaded dangerously close to that limit."

Hinton said he welcomed the coming trial of the accused killers of four American churchwomen as a sign of progress. But he added, "Salvadoran authorities, and you, the people of El Salvador, have tolerated serious excesses." He said he fully supports government efforts "to cut these abuses. Indeed, our future assistance is dependent on such improvement.

"If there is one issue which could force our Congress to withdraw or seriously reduce its support for El Salvador, it is the issue of human rights," Hinton continued.

The ambassador spoke on the eve of the arrival in El Salvador of a congressional fact-finding delegation including Sens. Patrick Leahy (D-Vt.) and Claiborne Pell (D-R.I.), both members of the Senate Intelligence Committee. A three-member House delegation is also due to arrive Friday.

Hinton, in a veiled reference to Salvadoran President Jose Napoleon Duarte, said that those who blame concern for human rights on a "conspiracy in the international press or on other outside influences are not facing the facts."

In a recent Newsweek interview, Duarte said the international media was giving a distorted picture of events his country, biased in favor of the rebels. "We are losing the fight with the guerrillas not only in the countryside, but in the pages of The New York Times and The Washington Post," he said.

Duarte spoke out again today, declaring the guilt of six of the guardsmen in the case of the American churchwomen. There are no formal charges yet against the suspects, who were arraigned yesterday in the small provincial capital of Zacatecoluca. A civil court judge has 48 hours remaining to evaluate the evidence and decide whether to proceed to trial.

Duarte, speaking in a television address to the nation, said that an investigation had concluded with "the full consciousness and moral certainty that those guilty of the murder of the four American women religious are the implicated."

Duarte repeatedly emphasized the thoroughness and detail with which the investigation was conducted to make the point that the six suspects are "the only and the true guilty ones."

There have been frequent accusations of a coverup in the case and suggestions that the murders may have been ordered by officials higher up in the armed forces than the currently detained former sub-sergeant.

"We are sorry that some entities and people would have taken advantage of this tragedy to benefit their particular wretched interests," said Duarte in an apparent reference to such criticism as he sat flanked by high military and civilian government officials.

As Duarte described the case, the chain of events leading to the murder began when National Guard corporal Margarito Perez Nieto and guardsman Alirio Elber Orantes Menjivar, stationed around the El Salvador International Airport, saw Sister Dorothy Kazel and lay worker Jean Donovan arrive in a white Toyota minibus on the afternoon of Dec. 2, 1980.

"The ex-guardsmen as Duarte described them said that the aforementioned women carried handbags in which they could hide weapons and because of that they decided to observe them from afar," Duarte said.

After Kazel and Donovan had picked up two other nuns at the airport and driven away, the corporal called up sub-sergeant Luis Antonio Colindres Aleman and told him about "the presence of these women and their suspicious attitude," Duarte said.

The sub-sergeant told the corporal to be careful, according to Duarte's account and when the minibus came back at about 5 p.m. the corporal and his friend went to the airport command post.

Shortly afterward the sub-sergeant ordered five of his men to dress in civilian clothes. They then dropped the corporal off at the control booth on the highway and the sub-sergeant told him to hold up all traffic except the nuns' Toyota.

When it came along a few minutes later, now with Kazel, Donovan, and Maryknoll Sisters Ita Ford and Maura Clarke inside, the sub-sergeant and his five guardsmen stopped them and searched their vehicle, Duarte said, without giving any idea what, if anything, was found. The sub-sergeant and four of his men all finally got into the nuns' van, taking them down a little dirt road on the way to San Pedro Nonualco.

"On arrival at a deserted place," Duarte said, the sub-sergeant "ordered a halt and having gotten out of the vehicle ordered his companions to sexually abuse some of the religious women, the same Colindres Aleman afterwards ordering them to shoot them dead."