Like a number of other Vietnamese refugees, the former captain in the Army of South Vietnam claimed to have seen an American in a reeducation camp long after the Hanoi government said all were gone.

Interviewed by U.S. officials in Indonesia last month after he had fled his homeland by boat, the refugee gave a physical description of the man he had last seen in June 1978. He said the American had taken a Vietnamese name, Nguyen Van Nam, and was employed taking care of the generators in a reeducation camp near the town of Yen Bai about 66 miles northwest of Hanoi.

A U.S. official here described the refugee's report as "very credible." In fact, he added, "I know exactly who it is. He's talking about Garwood."

Marine Private Robert Garwood, the former prisoner of war who opted to remain in Vietnam and returned to the United States in early 1979 to a court-martial for collaborating with the enemy, has been the subject of numerous "POW sightings" reported to the Pentagon.

Other alleged sightings are equally tantalizing at first, but not so easily dismissed. Yet none has provided any proof that any U.S. prisoners of war are still being held in Indochina.

The emotional issue, which came up again with the congressional testimony today of the ex-Green Beret POW-hunter James G. (Bo) Gritz, continues to occupy the three-man Bangkok office of the Hawaii-based Joint Casualty Resolution Center. And it is a continuing source of frustration to those dealing with the issue that Garwood--because of legal defenses and litigation pending over back pay--has never been officially debriefed.

The former marine has only hinted in press interviews that he knew of other Americans still in Vietnam while he was there.

At the same time that the presence of POWs cannot be confirmed, it cannot be finally refuted either. Thus there is still plenty of room for believers like Gritz to hew to their conviction that at least some Americans are still being held.

Gritz said this month after a series of forays into Laos that he is sure at least 10 U.S. POWs are still alive in Indochina. But U.S. officials here said that he admitted he had no solid proof.

In his testimony before the House Foreign Affairs subcommittee on Asian and Pacific affairs Tuesday, Gritz said he still believes at least 50 American POWs are being held in Southeast Asia but he could not provide Congress with any proof, The Associated Press reported.

Five separate Laotian "sources" gave his group evidence that Americans are alive, Gritz said. He promised to turn over his evidence to the committee but later admitted that it basically consisted of reports from his Laotian associates and was not of a "documentary" nature.

"I have the same evidence that might be presented by a clergyman to convince you that God exists," he said.

"Then your evidence rests exclusively on eyewitness reports?" he was asked by Rep. Stephen Solarz (D-N.Y.).

"That's correct, sir," Gritz answered.

When Gritz returned home, he said photographs taken by a Laotian working with his group would show a POW camp. But he told the committee that when he looks at the pictures now, "I find nothing there that would provide any usable evidence."

Nevertheless, Gritz's mission raised at least one family's hopes when it was reported that a Laotian resistance fighter working with Gritz had returned to Thailand from Laos with a questionnaire allegedly completed by a POW. Gritz, then in California, received the name of the purported prisoner in code in a telephone call from an associate in the northeastern Thai border town of Nakhon Phanom. The name was Morgan Jefferson Donohue, an Air Force first lieutenant shot down over Laos in September 1968.

According to a U.S. official here, Gritz asked his associate to keep the Laotian in Nakhon Phanom until Gritz could question him, but the resistance fighter disappeared across the border before the former special forces lieutenant colonel returned to Thailand in January.

The U.S. official suggested that the information about Donohue might have come from handbills that his father, retired Air Force Col. Vincent Donohue of Cocoa Beach, Fla., had distributed previously along the Thai-Laotian border in a fruitless search for his son.

The information about Donohue was apparently the "POW ID" Gritz said he had obtained and was trying to confirm in a reported trip into Laos in February.

Subsequently, Gritz has indicated that he feels partly responsible for Vietnam's decision to provide a team of U.S. officials with information this month on 12 Americans missing in the Vietnam and presumed dead. But a participant in the dealings with Hanoi on the MIA issue said it was unlikely that Gritz had anything to do with the development.

The U.S. officials were surprised that the Vietnamese never mentioned the Gritz raids during their meeting, the participant said.

The official said that in the March 10 meeting in Hanoi, the Vietnamese provided a list of 12 Americans on whom they said they had recently obtained information. All were airmen downed in North Vietnam, and the Vietnamese indicated they had recovered the remains of some but not all of them, the U.S. official said.

According to MIA specialists here, only two of the nearly 2,500 American servicemen unaccounted for during the Vietnam War currently are listed as missing, a classification that presumes they are legally alive and qualifies them for continued pay and promotions.

The rest are classified as killed-in-action, body not recovered. This means families are entitled to death benefits, but the listing occasionally has aroused sharp protests from relatives who charge that the government is trying to classify the problem out of existence.

In 1973 the Pentagon listed more than 1,300 persons as missing in action in Indochina. It is unclear how the armed services have arrived at the reclassifications.

For example, as of 1980 Morgan Donohue was still listed as missing. By 1982 he was officially considered dead. U.S. officials here do not know why.