U.S. officials engaged in high-level meetings with Vietnam to resolve the issue of missing U.S. servicemen are voicing increased annoyance with private American efforts to turn up living prisoners of war in Southeast Asia.

During the past year various private American citizens and interest groups stepped up their activities, the officials say, spreading misinformation about the existence of POWs in Vietnam and Laos, backing forays into Laos that have resulted in the "trashing" of U.S. aircraft crash sites and accusing the U.S. government of conspiring to cover up the issue of prisoners and missing in action.

"We're in the best position we've ever been in to resolve the issue," said a Reagan administration official closely involved in MIA matters. He expressed extreme irritation at some of the activities by these unofficial groups and said that they were hampering the administration's efforts.

A high-level U.S. delegation flew to Hanoi today to discuss the MIA issue with senior Vietnamese officials, including Foreign Minister Nguyen Co Thach, during a 24-hour visit. The seven-member U.S. team, led by Assistant Secretary of State Paul D. Wolfowitz and Assistant Secretary of Defense Richard L. Armitage, conferred with Deputy Foreign Minister Hoang Bich Son and is scheduled to meet Thach Tuesday.

Vietnamese officials have billed the meeting as a key element in helping to fulfill Hanoi's pledge last year to resolve the MIA issue within two years. Vietnam, which has been seeking diplomatic relations with the United States, has suggested that a U.S. technical office in Hanoi would advance resolution of the issue.

A senior U.S. official said Vietnam's two-year target would be difficult to meet. He said the U.S. side estimated that this would require 250 Americans in Vietnam to conduct surveys and joint excavations of crash sites. Washington would consider opening a technical office in Hanoi if the level of Vietnamese activity justified it, the official said, but it would not imply any diplomatic relationship and would be withdrawn at the end of the two-year period.

Although the Reagan administration is said to be making significant progress on the MIA issue, it has been dogged recently by what some officials see as the harmful activities of private Americans riding a wave of sentiment, inspired at least partly by the movie "Rambo."

Northeastern Thailand along the Laotian border has become known among U.S. officials as "Rambo country." One of them lamented "this rip-off Rambo business" that has drawn adventurers who dream of pulling off a POW rescue. Although many of them had been in business long before the Sylvester Stallone character popularized the idea, the movie has helped them raise money, the officials said.

The issue of live POWs appears to be an awkward one for the Reagan administration. Unlike the previous Carter administration, Reagan's bases its policy on "the assumption that at least some Americans are still held captive" in Indochina. But it is an assumption, in the administration's view, that some right-wing critics of Reagan may have taken too far.

The latest private activist to show up in Thailand is Jack Bailey, who heads "Operation Rescue." He has stated in fund-raising letters that it is "the only organization bringing out proof that Americans are still being held prisoners by the Communists."

Bailey appeared today at a press conference as an "adviser" to Son Sann, a 74-year-old Cambodian resistance leader who is fighting to retain control of his anti-Vietnamese guerrilla organization in an internal power struggle. Bailey stated afterward that he had established that 33 Americans were still being held prisoner in Laos, but did not specify the evidence. He reportedly has said previously that "there are 22 Americans still alive in Laos in three locations."

Bailey also said he has formed "reconnaissance teams" of Laotian anticommunist resistance fighters who have recovered MIA remains from Laos. An official analysis of one collection he produced revealed that they were a mixture of human and pig bone fragments, and no identifications could be made.

According to U.S. officials, an airplane crash site currently being surveyed in Laos by a team of U.S. specialists in preparation for a joint excavation has been picked over repeatedly by Laotians who then sell what they believe to be MIA remains to interested Americans.

Particularly irritating to the government, officials say, has been a suit filed by two retired Green Berets alleging a conspiracy and cover-up on the POW issue by the Reagan administration. The suit by retired Maj. Mark A. Smith and Sgt. 1st Class Melvin C. McIntire was filed on Sept. 4, 1985, in North Carolina's Eastern District Court against the president, the secretaries of State and Defense, the director of the Defense Intelligence Agency and their predecessors.

Smith and McIntire recently received nationwide attention for their case on the Phil Donahue television talk show, an appearance that an administration official said raised tens of thousands of dollars for their litigation fund.

In response, the Defense Department has declassified a two-page "fact sheet" that rebuts various allegations by Smith and McIntire. Among the claims by Smith and McIntire are that, on missions to Thailand since 1983, they uncovered proof that American servicemen were still being held in Laos a decade after the last U.S. prisoners of war were supposed to have been returned.

"All of the POW-related information obtained and reported by Major Smith and Sergeant McIntire with regard to Americans being held prisoner or missing in Laos has been determined to be either hearsay information from sources that could not substantiate the sightings, information that simply did not relate to Americans missing or unaccounted for in Southeast Asia, or information about an Army officer who died in captivity in 1961," the fact sheet said.

The officer who died, Capt. Walter Moon, was an adviser to the Laotian Army who was shot trying to escape Communist Pathet Lao guerrillas in an incident witnessed by fellow American prisoners, military sources said. However, they said, he has been the subject of a number of POW reports because of a Laotian biographical sheet, written while he was in captivity, that has circulated among Laotian refugees.

According to the Defense Department fact sheet, another POW-hunter, retired Army Special Forces Lt. Col. James G. (Bo) Gritz, obtained the original of the biographical document on Moon in 1984 "and submitted it to a congressional task force after the document had been tampered with so as to conceal the fact that it was dated in 1961." Gritz, 46, of Los Angeles, led a botched POW rescue attempt into Laos in 1983 and was briefly jailed by Thai authorities for illegally crossing the border between the two countries.

Gritz could not be reached for comment, but according to Lynn Standerwick, a close friend of his who was arrested with him, Gritz did not tamper with the document. She said he photocopied it in such a way that it also showed two photographs of Moon and a letter to his wife.

According to the Pentagon fact sheet, Smith and McIntire last visited Thailand in mid-1985 on a private mission to seek POW information at the behest of Rep. William M. Hendon (R-N.C.). They came up with the names of two Americans allegedly still held prisoner in Laos who had been the subjects of numerous refugee reports, the sheet said.

Many refugee reports have been based on advertising by concerned family members, some of it offering rewards and resettlement assistance in return for evidence on missing servicemen. One who has been reported frequently to be in captivity has been Morgan J. Donahue, an Air Force crewman who was involved in a midair collision with another U.S. plane over Laos in December 1968. U.S. authorities say that Donahue almost certainly was killed and that the reports have been generated by handbills distributed along the Thai-Laotian border by his father and brother.

U.S. officials also suspect that Vietnam has been manufacturing false "dog-tag reports" on allegedly missing servicemen based on material left behind when Americans evacuated Saigon in 1975. The purpose, the officials say, could be to discredit refugee sources, tie up intelligence resources and divide American public opinion.

About 75 percent of the Americans named in the dog-tag reports actually returned home alive, U.S. officials say, and 17 percent were killed and accounted for. Only 8 percent are listed among the 2,441 Americans still missing in Indochina.