The high command of the Salvadoran armed forces "is engaged in a conspiracy to obstruct justice" in the killings last year of six Jesuit priests, a key House Democrat said yesterday.

Rep. Joe Moakley (D-Mass.), chairman of the House Rules Committee and head of a special task force on El Salvador, made the accusations after hearing the findings of a staff investigation last week.

Salvadoran military officers have "withheld evidence, destroyed evidence, falsified evidence and repeatedly perjured themselves in testimony before the judge," Moakley said.

His attack is the first time a key House member has accused the most senior military leaders of deliberately blocking the investigation into the Nov. 16 killings of the six Jesuits and two women at the University of Central America in El Salvador.

The special task force's views are likely to be critical in determining the amount of military aid going to El Salvador.

A House-passed bill would provide $85 million in aid for the Salvadoran government in fiscal 1991, but would hold half of it back, to be awarded only if leftist rebels decline to participate in a cease-fire or peace negotiations. All the aid would be terminated if the Salvadoran government ends talks or fails to conduct a "serious and professional" investigation of the murders.

The Senate is expected to consider the House Salvadoran aid proposal next month.

The administration has agreed to place conditions on the aid in order to pressure the government to negotiate with the rebels, but it argues that 50 percent is too large a reduction. Administration officials have threatened that Bush will veto the bill if it contains the House-passed language.

A State Department official, asked about Moakley's statements, said the department "would not endorse every sentence that he's written" but agreed that "progress on the case has not been satisfactory."

Department officials said the administration had already moved to withhold $2 million in legal aid for judicial training and seminars after Salvadoran judges dismissed charges against most defendants in two other cases.

Senior department officials have been deeply unhappy with the lack of progress in the priests' murder investigation. "From the beginning, the administration has made it very clear that a satisfactory . . . prosecution of those responsible has been critical to this government," the official said.

Moakley said the investigation had bogged down despite the efforts of the judge, Salvadoran President Alfredo Cristiani and U.S. Ambassador William Walker. "I think Cristiani is well-meaning," Moakley said in an interview, "but the military is too strong."

The high command's goal, "from the beginning," the congressman said, "has been to control the investigation and to limit the number and rank of the officers who will be held responsible for the crimes."