Mai Xiong, a 29-year-old Hmong tribesman from Laos and self-described resistance fighter against the Communist government there, says he was splitting wood in his mountain village last October when a biplane flew overhead and sprayed a white substance. According to his account, when he breathed it he began to cough and sneeze.

"Two days later I began to get dizzy and started vomiting," he said in a recent interview in this refugee camp in northeast Thailand near the Laotian border. "I got some bleeding from the mouth, and I got diarrhea with blood. I was sick for almost two months. From October 1981 until I left in December, 25 people died. The pigs, the buffaloes and the chickens all died, and the rice and the corn died too."

Mai Xiong's story is typical in many ways of reports since 1976 by members of his tribe, who have been described by the U.S. government as victims of Soviet-backed use of chemical warfare by the Communist Laotian government and its Vietnamese allies.

The U.S. charges--leveled most forcefully in a 32-page report to Congress last month by Secretary of State Alexander M. Haig Jr.--have been controversial. U.S. critics in the press and the scientific community have questioned the reliability of the refugees' stories and expressed skepticism about the conclusiveness of physical samples that have been collected. The government's case still suffers from a failure to produce a "smoking gun": a rocket, shell or other piece of ordnance used to deliver poisonous chemicals.

Finally, skeptics have accused the Reagan administration of pointing at the Soviets as the source of the chemical weapons to help win approval for its own plans to start producing binary nerve gas weapons.

Nevertheless, accounts available along the Laotian border and in Bangkok offer strong support for the charge that poisonous chemicals have been used against the Hmong. The case for the allegations, though circumstantial, includes statements by Hmong refugees describing chemical attacks, interviews with Western doctors who have treated large numbers of refugees, reports by diplomats and military sources, and independent news accounts quoting Vietnamese and Laotian military defectors who claim they knew of use of chemical weapons.

The State Department report charged that guerrilla rebels in Cambodia and Afghanistan have also been attacked with chemicals. The Hmong, however, a simple, unsophisticated people, are considered the most credible of the different groups of victims. Unlike the Khmer Rouge in Cambodia, the Hmong have not made a concerted effort to wring propaganda benefits from chemical warfare reports.

Derek Bird, a British physician who came to Ban Vinai four months ago on a sabbatical, said he "quickly began to realize that their the Hmong refugees' stories held together . . . . As a scientist I try to be objective, and so far one has not found any conclusive clinical proof. But as a human being used to interviewing people, I have no doubt about the truth of their stories."

Western and Thai sources believe that the Laotian and Vietnamese governments are using chemical weapons to try to drive the Hmong out of Laos. Mai Xiong said: "I had to leave because we got attacked with chemicals too much."

A fiercely independent tribal people who live mainly in the mountains north of the Laotian capital of Vientiane, the Hmong largely supported the American side during the Vietnam War, fighting effectively against the Vietnamese and Pathet Lao Communists. They continue to put up scattered resistance today against the Laotian government and the estimated 40,000 to 50,000 Vietnamese troops stationed in Laos to support it.

Some Western diplomats believe that the skepticism in the United States and other Western countries about use of chemical weapons may partially account for apparent persistence by the Laotians and Vietnamese in using them.

Two weeks ago, an eight-member U.S. team of chemical and biological warfare experts visited Ban Vinai and reported that some Hmong among a group of 70 recently arrived refugees appeared to have suffered chemical attacks. The U.S. Embassy in Bangkok is continuing to receive new samples of purported chemical warfare agents from Laos and Cambodia, an embassy official said.

It is not clear exactly what kinds of chemical weapons are being used. Refugees have said they were attacked with substances in a variety of colors, modes of delivery and physical effects. Statements by the Hmong that cats and pineapples survived attacks that killed off other animal and plant life puzzle experts and underscore how little is known about the weapons.

But many details in the Hmong accounts remain the same and point to possible use of poisons known as trichothecenes, produced by fungi and belonging to a group of chemicals called mycotoxins. Scientists say such toxins cause dizziness, vomiting, diarrhea and hemorrhaging--all symptoms that have been repeatedly mentioned by refugees describing the effects of what they often call "yellow rain." In addition, traces of these poisons have been found on plants and rocks taken from areas in Southeast Asia after they were reportedly sprayed with chemicals.

Scientists say mycotoxins can cause horrible death by hemorrhaging, and the State Department said that chemical warfare in Laos has caused a minimum of 6,504 deaths since 1975. The claims are based on reports by survivors that cannot be independently confirmed, however.

U.S. critics point out that Hmong and Vietnamese made similar unverifiable assertions in the early 1970s that herbicides used by the United States in Southeast Asia had caused deaths or illnesses. Some of these claims were included in working papers published as part of a 1974 report by the National Academy of Sciences commissioned by the Pentagon.

"Perceptions about the results of chemical attacks can be wildly different when you're dealing with the Montagnards Hmong ," said Dr. Matthew Meselson, a biochemistry professor at Harvard University and chemical warfare expert, in a telephone interview from Cambridge, Mass. He headed a team from the American Association for the Advancement of Science that studied the effect of U.S. herbicides in Laos and Vietnam. "I'm not convinced anybody died from the herbicides, but people had the perception that there were deaths," he said.

Use of chemical weapons in warfare is barred under the 1925 Geneva protocol, and the governments of the Soviet Union, Laos, Vietnam and Afghanistan have strongly denied the American charges. The United States says the international accord did not cover the defoliants and tear gas it used in Vietnam. U.S. investigators believe that the defoliant Agent Orange and other American chemicals captured by the Communists in the 1975 takeover of South Vietnam are being used today by the Vietnamese or their allies in addition to the deadlier mycotoxins.

A United Nations group of experts assigned to conduct an impartial inquiry into the chemical warfare allegations said in a report last November that the evidence was inconclusive. Medical, diplomatic and Thai government sources familiar with the U.N. team's visit, however, called the panel's investigation "inadequate." They said the group was unprepared, did not stay long enough to carry out a thorough inquiry and lacked expertise in trichothecenes.

The signs are extensive that the Hmong have been attacked with chemicals. Since 1976, interviewers including journalists, doctors, Thai authorities and Western government officials have quoted Hmong as describing hundreds of chemical attacks and their results with a fair degree of consistency. Hmong refugees arriving in Thai camps rarely volunteer accounts of chemical warfare unless they are specifically asked, according to doctors who have treated them. In addition, the stories have come not only from resistance fighters but from women and children.

"I defy anyone to coach an illiterate 8-year-old Hmong girl to give a detailed description of a chemical attack that points to the use of mycotoxins," said a Canadian diplomat who has followed the subject closely.

Three Hmong tribesmen--Mai Xiong and two in the Nong Khai camp about 120 miles east of Ban Vinai--told this "As a scientist I try to be objective, and so far one has not found any conclusive clinical proof. But as a human being used to interviewing people, I have no doubt about the truth of their stories." correspondent several weeks ago that they had witnessed chemical attacks. The three were located by the Hmong commander at Ban Vinai and a Thai Catholic Relief Services worker at Nong Khai.

In Nong Khai, 49-year-old guerrilla Vaseng Lee said a plane on Feb. 15 fired four rockets that gave off yellow, red and white smoke. All who were exposed suffered dizziness and nausea and some fell down unconscious, he said, adding that Vietnamese troops arrived later and killed 45 of them.

In the same camp, 20-year-old Lue Vang said he saw a chemical attack in July 1981.

In addition to this first-hand testimony, four Western doctors who care for Hmong refugees said in separate interviews that they were convinced that the tribesmen had been attacked with chemicals.

"I began seeing a lot of atypical pulmonary diseases not attributable to anything else," said Dr. Richard Haruff, an American, during a recent return visit to Ban Vinai, where he ran a pulmonary disease unit for six months in 1980.

Some patients described chemical attacks when questioned, he said, and in 21 cases he was able to rule out other causes. He began calling these ailments "poison gas disease," saying it was a "presumptive diagnosis" because he could not medically prove with the limited facilities available that the diseases stemmed from chemical attacks.

Bird, the British physician, said his patients often said they had been gassed only after all tests for known diseases had proved negative and they were specifically asked if they had been exposed to chemicals.

Among the latest victims, he said, were a pregnant woman and a family of three who said they were sprayed by a plane Jan. 4 in the Phou Bia region of Laos. He said the married couple described symptoms including chest pains, coughing blood, vomiting and bloody diarrhea. He said the mother still has symptoms and the baby has a "blister-like rash that I can't identify as anything I know."

Amos Townsend, 51, an American doctor who has interviewed numerous Hmong refugees since April 1981 and was a medical coordinator at Nong Khai, said there is evidence that the attackers "are still experimenting with modes of chemical dispersion. They're using five or six different types of aircraft, different modes of delivery and very likely different types of chemicals."

A former U.S. Air Force flight surgeon who once studied biological and chemical warfare at Fort Detrick, Md., Townsend said that interviews with a number of Hmong refugees indicated that the Communist authorities often visited villages after they had been hit "to see how many people died and how many were sick."

"I have to consider Laos an open laboratory," he said.

Dr. Gideon Regalado of the Philippines said he was certain chemical attacks were continuing. He also criticized the U.N. team of experts that visited Thailand last November to investigate the chemical warfare charges.

"They came here in a hurry and did their questioning in a hurry," said Regalado, who was interviewed by the U.N. team at Ban Vinai.

The team was led by Maj. Gen. Esmat Ezz, head of the Egyptian Army's scientific research branch. It included a Filipino lieutenant colonel from his country's military ordnance and chemical service, a Peruvian professor of tropical medicine and a Kenyan orthopedic surgeon. They were accompanied by a secretary, a French consultant and two other U.N. officers, an Iranian and a Pole.

According to Townsend, the Pole became "very upset" during an interview when Townsend explained his findings to the panel and suggested the chemical weapons came from the Soviet Union. The Pole jumped to his feet and, warning against "wild allegations," said the Soviets had told him "on their honor" that they had never used chemical weapons or sent them to Southeast Asia, Townsend said.

Members of the group became even more upset at the end of the interview when Townsend handed them two vials of blood from a newly-arrived Hmong refugee and another vial containing yellow-spotted leaf samples from Cambodia, the American said.

"It was a little explosive," Townsend said. "Gen. Ezz said the group really did not come for specimens and would rather not have them," but decided it had no choice but to accept the samples.

Regarding the credibility of interviewees who said they'd been attacked with chemicals, the U.N. team said that it "did not find any reason to doubt the integrity of those who were interviewed" but that "it could not overlook the fact that it was difficult to determine the objectivity of alleged victims or witnesses."

The panel lamented in its report that it was able to visit only Thailand, because the governments of Afghanistan, Laos and Cambodia refused permission to investigate sites of alleged chemical attacks. According to Thai military sources, however, the group refused an offer by the Khmer Rouge government, which still has U.N. recognition, to take the team into territory it controls along Cambodia's western border with Thailand.

A U.S. diplomat said the U.N. team was "deliberately creating a conundrum . . . .They know they're not allowed into Laos and Afghanistan, and they refused to go into Cambodia. They're essentially defining the problem out of existence."

Asked for a response to the charges, U.N. spokesman Joe Sills said in a written statement in New York: "There is no reference in the panel's report to any offer having been made to the group by Khmer Rouge sources to visit the sites of alleged chemical weapon attacks." The statement said the U.N. team decided against visiting a refugee camp in Thailand near the border, where many alleged Cambodian victims of chemical attacks are staying, because the group felt it was not safe to go there.

"Within the constraints imposed by an ad hoc exercise of this nature, the group undertook a thorough investigation," the U.N. statement said. It said the team stayed in Ban Vinai camp until it had interviewed "all alleged victims, eyewitnesses and medical personnel who were made available for the purpose," that it repeatedly asked for samples and that the specimens provided by Townsend were still undergoing scientific examination.

A Thai source who accompanied the team said one of the more interesting interviewees was barely mentioned in the report: Quan Nguyen, a 36-year-old artillery captain from Vietnam who defected to Thailand in January 1980 and who said that chemical shells were being used in Cambodia.

According to the Thai source, the defector told the panel that three types of shells were used--a nonlethal irritant, a lethal but nonpersistent chemical, and a lethal and persistent chemical--but added that he himself had only used the first. Quan Nguyen was also quoted as having told the panel that two types of artillery were used to fire the shells, Soviet-made 130-mm pieces and captured U.S. 105-mm guns, and that the Soviets provided shells of both calibers. The chemical rounds had markings in Russian, the defector was quoted by the Thai source as saying.

The U.N. report summarized Quan Nguyen's account as follows: "He says he knows of three types of chemical weapons: tear gas, dissolving chemical and stronger, destroying chemical. He had used only tear gas." The defector is believed to be still at a holding center near the Cambodian border.

Another key defector, 35-year-old Laotian Air Force Pilot Touy Manikham, had left for resettlement in New Zealand 10 days before the team arrived in Thailand. In a detailed article in the Hong Kong-based Far Eastern Economic Review in January, he was quoted as saying that he flew an L19 spotter plane from which he fired mysterious missiles containing an unknown gas at Hmong villages from 1976 to 1978.

According to the article, Touy described attacks on targets in Xieng Khouang Province in which he was accompanied by a high-ranking Pathet Lao officer, with orders to leave the area immediately without the usual damage-assessment pass. The missiles exploded above the ground, he was quoted as saying, releasing billowing clouds of yellow and light red smoke, with a little blue-white smoke at the point of detonation. Those are the same colors listed by Hmong tribesman Vaseng Lee when he described a February attack in Laos.

When asked about skepticism in the West about the reported use of chemicals, Chong Moua Lee, the deputy Hmong commander at Ban Vinai, said "it's not my problem." After a pause, he added: "But I'm sorry that my people have been killed by gas for many years and not many countries believe it."