The United States financed and helped train a secret force of mercenaries who went into Laos earlier this month to determine if Americans were imprisoned in a jungle camp there, as covert overflights by satellites and spy planes had suggested.

The reconnaissance force returned a few days ago without any evidence that Americans were in the camp, chilling the hopes that the earlier photographic intelligence had raised in the minds of some Pentagon officials.

The team of Asians, believed to be Laotians, had been issued cameras to take pictures of the camp's inhabitants to supplement the overhead photography collected by the CIA and Defense Intelligence Agency, sources said.

The sources added that photo interpreters scrutinized all the photography last weekend and concluded that there were no Americans in the jungle camp.

Asked about the mission, Col. Ronald A. Duchin, chief of the Pentagon news branch, said last night, "We do not comment on any special operations of this kind."

In another statement personally approved by Defense Secretary Caspar W. Weinberger, the Pentagon said the following when queried by The Washington Post about efforts to rescue any Americans still being held captive in Southeast Asia:

"There has been a steady flow of reports about Americans being held against their will in Southeast Asia since the flow of Vietnamese from Southeast Asia began. The U.S. government regularly, and with great care, had and will continue to check the validity of each of these reports. The United States can be expected to take appropriate action if any of these reports proves to be true. Top priority effort will continue to be assigned to investigate these efforts."

The families of servicemen missing in the Vietnam war have continued to believe that some of their husbands or sons are still alive and are being held captive in Southeast Asia. They have pressured the government repeatedly to try to locate the men and rescue them.

The National League of Families of American Prisoners and Missing in Action in Southeast Asia has complained that the U.S. government has failed to press hard enough to find missing Americans.

By the Pentagon's count, there are 2,528 Americans unaccounted for in Southeast Asia, including 1,237 listed as missing in action. According to the Pentagon, 560 U.S. airmen went down in Laos, with 295 of them believed killed in crashes.

Of the other 265, 14 have been released alive as prisoners of war through Hanoi. Laos also has sent the United States the remains of two other American servicemen.

The mystery over what happened to the unaccounted-for Americans in Laos has lent credence to reports that some are still being held prisoners there. The Seattle Post-Intelligencer in February quoted Sayfa Phounsavan, a former Royal Lao air force pilot, as saying on the basis of refugee reports that, "There are today 40 or 50 Americans, most of them pilots, who were shot down over Laos during the war, being held prisoner there now."

Presumably on the basis of such reports, the U.S. intelligence community intensified its overhead spying on Laos. Sources said satellite photos showed shadows on the ground in the Laotian camp that some interpreters believed were too big for Asians.

Furthering this theory were pictures of tools which, to photo interpreters, looked too long for Asians but just right for Caucasians.

Also, the photos suggested that the word B52 had been spelled out with bodies or logs within the camp during one pass of a satellite or SR71 Blackbird spy plane. One theory was that this was a signal from desperate American POW's imprisoned as slave laborers in the jungle.

Other overhead pictures, sources said, showed what looked like a guard tower and earthen barriers to keep people in what appeared to be a prison compound.

However, this overhead evidence was not conclusive enough to justify a risky military action like the Iranian rescue attempt. Even if there were Caucasians in the compound, as the photos of shadows and tools indicated, they might be Soviet advisers there of their own free will. Rescuing them would be a tragi-comedy.

It was this lack of hard evidence that spawned the U.S. effort to recruit and train a team of Asians to go overland from Thailand into Laos to get a close look at the captives. Sending in an American team would have caused an international storm of protest if it were detected.

Just how the team of Asians was recruited and how much they were paid could not be learned. But sources indicated that the fee was handsome and the training extensive.

The Asian mercenaries apparently ran into difficulties shortly after they got into Laos, causing them to delay their mission, perhaps by picking an alternate route. There were hints, but no hard information, that the team believed it had been detected.

The team, believed to be 20 to 30 men, was said to have gotten close enough to the camp to study its inhabitants and report back that no Americans or any other Caucasians were in the compound. Apparently the mercenaries managed to take some pictures as well, although sources were guarded in discussing what evidence was brought back to Thailand. s

Rear Adm. Jerry O. Tuttle, a deputy director for intelligence at the Pentagon's Defense Intelligence at the Pentagon's Defense Intelligence Agency, was the Washington field general for the secret mission into Laos, sources said. The U.S.-sponsored foray came after POW families sought to launch such a mission on their own.

This earlier, private plan was aborted after the group ran out of money. Twenty-six men, a group made up large of ex-special forces soldiers and Vietnam combat veterans, assembled near Leesburg, in central Florida, for the privately financed POW rescue mission into Laos.

Their goal was to liberate at least one American prisoner, among 29 they believed to be held against their will in the jungle camp. They wanted "to show the world" that Americans are still being held captive in Southeast Asia, said ex-Green Beret James (Bo) Gritz, a highly decorated Vietnam combat veteran and retired Army lieutenant colonel who works in overseas operations for Hughes Aircraft in Los Angeles.

Gritz had been working to plan the private rescue mission for more than two years, a plan backed by money and moral support from several board members of the National League of Families.

"This is the opportunity of a lifetime," said retired Sgt. Maj. Fred Leenhouts, 42, a high school ROTC instructor from St. Petersburg who said he dropped everything to participate. "Can you imagine some guy over there caged up like an animal all these years? When I got called for the mission I got goosebumps all over my arm."

Ann Griffiths, president of the National League of Families POW/MIA and mission organizer, made several trips to Florida to pass along intelligence information and target data she said she had gleaned from her sources in government.

Griffiths sits on the Interagency Task Force on POW/MIAs, which includes members of the State Department, the White House staff, congressional staff and the Department of Defense.

She said she "would be my life" that Americans are still being held captive in Southeast Asia, based on highly sensitive Defense Department information of which she had knowledge. She said she was aware of the U.S. spy satellite photos with the word "B52."

George Brooks, a board member of the National League of Families and a Connecticut engineer, said in an interview that he had donated $20,000 to the mission, code-named "Project Velvet Hammer." The sun fell far short of the $380,000 Gritz estimated that his men would need to buy round-trip plane tickets to Bangkok, their launching point, AR180 machine guns with silencers, mosquito repellent, gold bullion to bribe the forces they might encounter in the jungle and other equipment.

The Gritz team, originally based for training at a cheerleading academy, later moved to the 7,000-acre estate of a central Florida tree farmer, Charlie Overstreet. Then, their rocky fund-raising efforts turned sour.

Emissaries from their group met with potential donors ranging from Texas millionaire-industrialist H. Ross Perot, who paid Gritz several hundred dollars in seed money, to representatives of Federal Express in Memphis and a group of unidentified Florida financiers. But the potential donors worried that they might be breaking the law by backing a group that did not enjoy the official sanction of the U.S. government, said Gritz, and declined to contribute.

The fund-raising fiasco, along with an ABC-TV series on American POWs-MIAs that reported that unidentified groups were training for rescue missions, and premature disclosure of their plan by The Orlando Sentinel Star, led Gritz to disband the mission.

But Gritz still insists his plan is visable, and says he and his men are ready to regroup should funds materialize. The men planned to fly to Thailand and set up a rice giveaway tent in refugee camps along the Thai-Laos border as a cover for the mission.

With help from Lao resistance, they would sleep by day and slip through the jungle by night, 50 miles to the target. A photographer, Tom Zimberoff of Los Angeles, would then snap photos of POWs with an 1,800mm lens, as backup to the one prisoner they planned to snatch.

At the moment they had the prisoner, Gritz would radio a team member in Thailand, who would then telephone Griffiths in Washington. Griffiths would then officially request a rescue by Navy helicopters from any aircraft carriers nearby. If the U.S. government refused, Gritz would rely on his own private rescue helicopters owned by the Thai police.

The Post learned about the secret, officially-backed mission into Laos before it was launched, and agreed to withhold publishing the information as long as there was any chance of Americans being identified and rescued. Top officials have now firmly concluded that there are no Americans in the camp in Laos, defense officials said.

The Defense Department made no request yesterday for The Post to continue to withhold publication of what it had learned about the foray. In fact, some Pentagon officials saw the story as helpful to the Reagan administration's effort to convince prisoner of war families that it is determined to pursue every avenue to rescue any Americans still held captive in Southeast Asia.