A former National Guard sergeant has testified before Salvadoran judicial authorities that one of his men confessed soon after the crime that he was "the problem" in the December 1980 assault and murder of four American churchwomen.

The testimony could help advance a case that has become one of the major targets of criticism by congressmen charging that the U.S.-sponsored Salvadoran government fails to live up to human rights and judicial standards set as conditions for continued military aid.

Dagoberto Martinez Martinez, now retired and living in Los Angeles, gave the testimony implicating Cpl. Luis A. Colindres Aleman in a deposition Saturday at El Salvador's international airport to a panel headed by Judge Bernardo Rauda Murcia, according to a court transcript.

Although Martinez provided similar evidence Feb. 10, 1982, to the FBI, Salvadoran judicial authorities had refused to accept it as evidence against Colindres in the courts here. The repetition of his testimony before the Salvadoran judiciary thus provides admissible evidence in the long effort by U.S. diplomats and lawyers for the women's families to obtain prosecution of Colindres and five other suspects jailed in the case.

The testimony by Martinez was considered here to be a major advance to get his account into the official court record. The lawyers have charged for months that Salvadoran authorities are seeking to avoid punishing those who abused and shot the American churchwomen.

R. Scott Greathead and Michael H. Posner, who have been pushing for prosecution on behalf of the families, said in a July 14 letter to Langhorne A. Motley, assistant secretary of state for Inter-American affairs, that getting the Martinez testimony in acceptable form was at the top of their list of what should be done to advance the case. The two, from the Lawyers' Committee for International Human Rights in New York, wrote Motley after a visit here earlier last month during which they conferred with Salvadoran judicial authorities and diplomats from the U.S. Embassy.

"On the basis of our visit, we continue to be deeply concerned that no one will ever be successfully prosecuted for these crimes," they said. "More than 2 1/2 years after the murders, the Salvadoran prosecutors still have not completed a number of basic investigative steps that are critical to a successful prosecution in this case."

The four women--Ita Ford and Maura Clarke of the Maryknoll order, Dorothy Kazel of the Ursuline order, and Jean Donovan, a lay worker--were killed Dec. 2, 1980, shortly after leaving the international airport. Their bodies, some of them half-clothed and all shot through the head, were found in a common grave 40 hours later, about 20 miles away.

The murders, which occurred in an area heavily patrolled by National Guard troopers including Colindres, quickly became a major issue between the U.S. and Salvadoran governments. Soon after they occurred, Defense Minister Carlos Eugenio Vides Casanova, then head of the National Guard, called his men to a conference in San Salvador and asked anybody who knew about the killings to let him know. During a recess in that conference, Martinez testified, Colindres walked up to Martinez and said: "Sergeant, the problem with the nuns is me."

Martinez said he advised Colindres to go to Casanova but never asked again whether he actually did nor inquired further about Colindres' statement because he "did not know if it was true or false." According to a transcript of the testimony, Martinez never went to Vides Casanova with what he knew, the testimony said, because he was afraid "he could have upset the investigation" then going on.

Colindres and five other guardsmen were arrested on the basis of other evidence soon afterward. Prosecutors have been gathering evidence for months. Posner and Greathead charged that the prosecutors have failed to develop the evidence, a charge echoed privately by U.S. diplomats here who say the judiciary long has been afraid to pursue crimes committed by security forces.

Martinez, meanwhile, went to the United States in May 1981. Diplomatic sources here said he is an illegal alien who has applied for political asylum. He was brought to the airport by the FBI and the State Department to testify, they said.