After years of enmity between prowestern Thailand and communist Laos, a festering border dispute has spawned a diplomatic war of the white papers.

In the latest salvo, the Thai Foreign Ministry issued a 33-page document last week charging that its landlocked Indochinese neighbor has become a "willing servant" of Vietnam and allows "increasing numbers" of Vietnamese settlers to move there. Among these settlers, the white paper said, are many former soldiers from the estimated 45,000 Vietnamese troops stationed in Laos.

The charges came in response to a Laotian white paper issued in September following a border dispute over three villages that Laos claims were illegally occupied by Thai troops last year. The Laotian white paper accused Thailand of harboring "expansionist and hegemonist ambitions" against Laos dating back centuries.

Further complicating matters have been reports -- so far unconfirmed -- that a team of American adventurers has arrived in Thailand with the aim of illegally crossing into Laos to search for U.S. servicemen missing in action during the Vietnam War.

Thai police near the border were reported on the lookout last week for former U.S. Special Forces colonel James (Bo) Gritz, who led an abortive attempt to search for American prisoners of war in Laos in March 1983. The attempt ended in a fiasco when Gritz and other veterans were chased back across the border by Laotian troops, then were arrested, tried and expelled by Thai authorities.

U.S. officials said there was no evidence that Gritz had returned to Thailand, but there were reports that other Americans could be planning a similar foray to search for POWs.

The Gritz mission upset official U.S. efforts to negotiate with Laotian authorities permission to excavate the site of a U.S. military plane crash near Pakse in southern Laos. Thirteen American crewmen still are missing from the December 1972 crash.

U.S. officials now hope to excavate the site this year following Laotian agreement in principle in 1984, but they are concerned that any illegal private forays again would set back U.S.-Laotian cooperation on the MIA issue. Of around 2,500 Americans missing in the Indochina conflict and presumed dead, about 560 disappeared in Laos.

The communist government that took power in Vientiane in 1975 has returned only three sets of remains, one of which was identified as American. Others have been turned over privately for a price by Laotian anticommunist rebels.

There has been no indication that the official U.S. efforts have been affected by the deteriorating Thai-Laotian relations, but much is believed to depend on the attitude of Laos' Vietnamese mentors.

Thai authorities say the border problem has been fueled behind the scenes by Hanoi, since Thai troops were withdrawn last October from the three disputed villages of Ban Mai, Ban Klang and Ban Sawang.

In a recent statement, the Laotian Foreign Ministry charged that Thai troops launched a series of attacks earlier this month across the border near the disputed villages "to extend their occupation of Lao territory" but were repulsed by local militia. The statement claimed that "Thai troops heavily pounded the area with all-caliber artillery fire, including 155-mm howitzers."

Thai Foreign Ministry spokesman Sawanit Kongsiri denied that any such actions occurred and said that the Laotian charges were being manufactured by Hanoi to distract attention from the Vietnamese dry-season offensive against Cambodian resistance settlements on the Thai-Cambodian border.

"Laos doesn't want to let the border dispute die down because the Vietnamese don't want to let it die down," Sawanit said.

In the Thai view, pervasive Vietnamese influence in Laos is a major factor behind a steady flow of Laotian refugees into Thailand. The white paper said that from 1975 to November 1984, more than 286,000 refugees crossed into Thailand from Laos out of a population of about 3 million.

The paper said Hanoi's aim was "to place Laos under the total control of Vietnam" and realize Ho Chi Minh's dream of an Indochinese federation uniting Vietnam, Cambodia and Laos.

The white paper said Vietnamese "have infiltrated into every aspect of Laos' social fabric" and that advisers from Hanoi were effectively in charge of the government. Moreover, it said, Vietnamese settlers were causing hardships and resentment among Laotian residents, who were powerless to resist the influx.

The paper gave no figure for Vietnamese settlement in Laos, but spokesman Sawanit said the settlers numbered about 60,000. He said these were new settlers rather than Vietnamese who had lived in Laos before the communist takeover.

Western Laos watchers here expressed skepticism about the Thai charges, however. MacAlister Brown, a visiting American professor from Williams College who visited Laos in October, said he saw no evidence of such a large Vietnamese influx.

"I don't know where they'd settle," he said. "Laos doesn't have the available land." He added that as a means of controlling Laos, Vietnamese settlement was no more effective than the stationing there of Vietnamese troops. Brown also said Vietnam's idea of an Indochinese federation, abandoned decades ago, was a "nonstarter" and unnecessary to Vietnam's control of Laos and Cambodia.

A western diplomat, expressing similar views, said Vietnam was not seeking any formal federation but wanted to maintain control over the two countries' communist parties and armed forces.

"What the Vietnamese want are satellites," the diplomat said, "very similar to what the Russians have in eastern Europe but with even more conformity."