ZACATECOLUCA, EL SALVADOR, JAN. 8 -- A judge ruled today that five convicted killers of four American churchwomen could not be freed under a sweeping amnesty declared as part of a Central American peace plan.

"The court finds the appeal for amnesty to be without grounds and therefore rejects it," said Consuelo Salazar Alvarenga de Revelo, judge in the 1st Criminal Court of Zacatecoluca, 40 miles southeast of San Salvador. She said the ruling could not be appealed.

The five National Guardsmen were convicted of the Dec. 2, 1980, rape and murder of Jean Donovan, a lay worker; Dorothy Kazel, an Ursuline nun; and Ita Ford and Maura Clark, Maryknoll nuns.

The murders, which came at a time of great upheaval in El Salvador, helped focus U.S. attention on the nation's civil war, and the Carter administration briefly cut off aid to the government because of the killings.

The ruling was a victory for the United States, which last week saw the convicted killers of two American land reform advisers go free under the amnesty.

Following the release of the killers of the land reform advisers, the United States threatened to cut its $9 million judicial aid package.

President Jose Napoleon Duarte implemented the amnesty Nov. 5 as part of a regional peace plan.

The amnesty, which has been harshly criticized by human rights groups, freed about 470 political prisoners, and made it impossible to prosecute people involved in death squad activity or military abuses against civilians.

Judge Alvarenga de Revelo said the killing of the nuns was not a political crime and that, therefore, the amnesty did not apply. She said the United States did not pressure her on her ruling.

"We have not yet seen the decision of the judge, but if that is the decision we are pleased," a U.S. Embassy spokesman said. "This supports our position from the beginning."

On May 23, 1984, Luis Antonio Colindres Aleman, Carlos Joaquin Contreras Palacios, Francisco Orlando Contreras Recinos, Daniel Canales Ramirez and Jose Roberto Moreno Canjura were sentenced to 30 years in prison, the country's maximum sentence, for aggravated homicide and robbery in the case.

None of those convicted was an officer, and questions have lingered about who ordered the murders.

In May 1983, the State Department appointed Judge Harold R. Tyler Jr. of New York to head an independent investigation into the killings.

In his report, Tyler said it was probable that Col. Roberto Monterrosa, head of the government's official investigation of the crime, was "aware of the identity of the killers and, further, that he participated in the cover-up by purposely failing to provide Colindres Aleman's fingerprints to the United States for analysis."

"We believe as well that it is quite possible that Col. Carlos Eugenio Vides Casanova, then head of the National Guard and now a general and minister of defense, was aware of, and for a time acquiesed in, the cover-up," the report said.

Vides Casanova is still defense minister and is often praised by U.S. diplomats.

The other major case involving the killing of Americans is still pending before a military judge.

On June 19, 1985, four U.S. Marines and two American computer consultants were gunned down at two sidewalk cafes by leftist rebels.

The three men arrested for the killings have applied for amnesty. A lower court granted the amnesty, but the decision was appealed by the attorney general and the U.S. Embassy.