Col. Jose Guillermo Garcia, the Salvadoran defense minister, today confirmed that six members of his armed forces had been provisionally arrested on suspicion of having murdered four U.S. churchwomen last December.

After weeks of government denials of similar reports, Garcia, flanked by six of his fellow ranking officers, admitted at a press conference here that the six military men had been under detention since April 29.

The acknowledgment came after former ambassador to El Salvador Robert White said in the United States that the Reagan administration had known for weeks that six members of the security forces were suspected of the killings. White's declaration was followed by U.S. officials' accounts yesterday in Washington to the effect that they knew who the killers were but were unsure that evidence collected by the FBI was sufficient for conviction.

[In Washington, State Department spokesman David Nall said of Garcia's announcement of the arrests, "This action demonstrates that the investigation is making progress and shows the determination of the government of El Salvador to act against wanton violence, whatever its source."]

Garcia refused to name the suspects or to identify their unit. He said, however, that they were neither "sergeants nor officers," and his constant referral of specific questions about the men to Col. Eusebio Vides Casanova, the head of the National Guard, tended to confirm the earlier repots that the suspects were National Guardsmen.

Garcia insisted that the men were being detained "provisionally" at a military barracks here pending collection of further evidence that would be needed to establish whether there were; sufficient grounds to bring the men to trial.

The six men were being held in connection with the Dec. 2 killing of four U.S. missionaries -- nuns Maura Clarke, Ita Ford and Dorothy Kazel and lay worker Jean Donovan. The four women were last seen alive at San Salvador's Cuscatlan International Airport, shortly before nightfall.

The following day their white Toyota van was found burned and with its license plates removed on a road northwest of the airport. A day later the four churchwomen's bodies were found in an unmarked grave near the villiage of Santiago Nonualco, 20 miles from where the destroyed van was found.

From the beginning it had been suspected that six National Guardsmen, among El Salvador's most feared soldiers, had been responsible, since they were known to manning a roadblock just beyond the airport from which the churchwomen were driving that night.

Col. Garcia insisted today that the six men were only being held on "suspicion," based mostly on the circumstantial evidence of their haivng been at the roadblock, pending more detailed incriminating evidence that he insisted the government was still awaiting from the FBI. Its investigative services were brought into the case at the specific request of the embarrassed civilian-military junta that rules El Salvador.

Two weeks ago, when the first reports of the six suspects were leaked, the FBI stated that its "evaluation" report of its investigations into the case does not "refer to suspects, does not point a finger or name those responsible."

That evidence was believed to have hinged on the matching of fingerprints of the suspects with those taken from the burned van. Judicial officials here have indicated that the first five sets of fingerprints submitted to the FBI did not match up with those found on the van. A second batch of fingerprints are alleged by Salvadoran officials to have been sent to Washington, but U.S. officials here have neither confirmed that fact nor revealed what the results might have been.

Col. Garcia said that not only had fingerprints been sent to the FBI for checking but that the suspects' weapons had also been turned over to the U.S. Embassy so that the FBI could conduct ballistic tests.