Communist Khmer Rouge guerrillas battling the Vietnamese occupation of Cambodia have produced what they say is fresh evidence that Hanoi is continuing to use chemical weapons in the three-year-old war.

Interviewed at a makeshift hospital at this jungle camp near the Thai-Cambodian border, Khmer Rouge guerrillas described a Vietnamese gas attack Feb. 13 about 10 miles south of here. Khmer Rouge officials displayed a gas mask that they said had been taken from a dead Vietnamese soldier March 1 after a battle for a hill about 12 miles to the southeast.

According to U.S. officials in Bangkok, the mask is the first piece of equipment related to Hanoi's alleged use of chemical warfare to be found in Indochina. As such, one official said, it represents a "significant," although not conclusive, contribution to the growing body of evidence that Vietnamese forces are using chemical weapons in Cambodia.

Khmer Rouge officials said the mask, made of gray rubber and plastic, was captured when guerrillas attacked a Vietnamese unit at Phnom Poch.

According to Long Norin, a senior Khmer Rouge official here, the Vietnamese have sometimes fired artillery shells containing poison gas at Khmer Rouge positions to evacuate besieged Vietnamese units.

Widely vilified for the brutality, mass executions and destruction of its 1975-79 rule in Cambodia, the ousted Khmer Rouge government has never been given much credibility. However, in this instance Khmer Rouge accounts of the use of chemical weapons by the Vietnamese appeared consistent with information from Western and Thai sources.

For example, intelligence sources in Bangkok have reported that the Vietnamese distributed large numbers of gas masks earlier this year to units fighting in western Cambodia.

There were no markings on the mask displayed here other than a series of numbers, and its origin could not immediately be determined positively. However, a U.S. source who examined it said it "could very well be a Soviet mask," judging by the plastic star symbol over the air-intake filter.

The Vietnamese Embassy in Bangkok had no immediate comment on the gas mask. But Hanoi recently accused Thai forces of firing poison gas shells into Cambodia, a charge denied by the Thais and seen by some Western diplomats as a possible pretext for the Vietnamese distribution of the masks.

Up to now, charges that Hanoi has used chemical weapons have been based on plant and blood samples said to contain traces of mycotoxins of the trichothecene group, chemical agents that can cause death by internal bleeding, diarrhea and vomiting.

The samples have been collected in Cambodia and Laos, where guerrillas are battling Hanoi-backed governments kept in power with the help of Vietnamese troops.

The most serious allegations have concerned "yellow rain," a highly toxic substance dropped from aircraft and capable of causing violent, bloody death within minutes.

In the latest sample from a suspected "yellow rain" attack, Khmer Rouge officials have collected what they say is a contaminated leaf from a battle zone across the border from the Thai village of Pong Nam Ron. According to the Khmer Rouge, the substance was dropped Feb. 26 in an attack by two Vietnamese planes. The sample has been forwarded to the U.S. Embassy in Bangkok and is to be sent to the United States for analysis.

Also being sent are new blood samples from some of the 17 Khmer Rouge guerrillas said to have come under a Vietnamese gas attack during a battle at Tuol Chrey on Feb. 13.

According to the leader of the unit, Prak Reth, the battle began in the morning and lasted all day. Then at 6:30 p.m., he said, the Vietnamese fired 105 mm shells containing poison gas.

"At first I had trouble seeing, like there was soap in my eyes," he said through a Khmer Rouge interpreter. "I also had difficulty breathing. My nose felt inflamed, like it was burning. Then I started vomiting. I vomited from 6:30 until morning. I could run about 500 meters [580 yards] before I fell down. Besides nausea, I also had diarrhea until morning."

Prak Reth said he could not see the gas or tell how many shells had struck because it was night. He said none of the guerrillas died from the gas.

Prak Reth was among six guerrillas who gave blood Wedenesday to a private doctor collecting samples on behalf of the U.S. government. The doctor, Amos Townsend, has been instrumental behind the scenes in investigating alleged chemical warfare incidents in Laos and Cambodia. He rushed to this jungle camp after learning of the presence of the 17 purported gas victims.

A former U.S. Air Force colonel, Townsend, 51, also went into Khmer Rouge territory in Cambodia in October to collect blood samples from guerrillas. The samples were later used by the U.S. government to bolster its allegations against Vietnam, although officials refused to say how and from whom the specimens were collected.

In the latest airing of the charges, the U.S. ambassador to the United Nations, Jeane J. Kirkpatrick, Wednesday again referred generally to blood samples collected in Cambodia last fall. In a letter to the U.N. secretary general, Kirkpatrick said the samples and survivors' accounts provided "strong circumstantial evidence that trichothecenes were used as chemical agents in yet another chemical attack in Southeast Asia."

Specifically, the U.S. government said an independent analyst was able to "tentatively identify" in samples from two of the nine gas victims a metabolite, or breakdown, of the T2 toxin said to be a component of "yellow rain."

According to Dr. Townsend, the alleged gas victims seen Wednesday "looked very similar to the nine we drew blood from last October." Although there was no sign of any wounds, "they just looked very listless and fatigued," he said.

In fact, one of 17 guerrillas said he had also been incapacitated by poison gas in the earlier attack, which he said occurred Sept. 19 at Takong.

The guerrilla, O Rin, 26, said the gas came from a land mine triggered by a fellow soldier.

O Rin said he suffered severe vomiting and diarrhea and was hospitalized for four months. He said there were more than 80 other "heavy cases." The boyish-looking guerrilla, already a 10-year veteran of the Khmer Rouge, said he returned to combat in January and was struck down by gas a second time last month. Affected less seriously this time, he expects to be released in another 10 days.

According to U.S. officials, the Vietnamese appear to be using several different chemical agents, possibly including some substances other than the mycotoxins that officials believe are produced in the Soviet Union. Some of the substances apparently are not designed to kill but to incapacitate, injure and terrorize, thus leaving survivors who impede evacuations, clog hospitals and drain resources, the officials said.

So far neither the Khmer Rouge nor Western investigators have been able to find what one Bangkok-based diplomat said would be the real "smoking gun" in the case: a piece of Vietnamese chemical ordnance such as a gas canister or artillery shell. Khmer Rouge officials said the Vietnamese have thwarted the search by mixing poison gas shells with a preponderance of regular shells in their barrages. However, the officials said Khmer Rouge units now were making special efforts to find chemical ordnance.

Despite the international furor over the allegations, Townsend said he was not surprised at reports that the Vietnamese are continuing to use chemical weapons in Cambodia.

"The advantage of gas is that if you know people are out there somewhere, you don't have to aim so well," he said. "It's just too useful in a jungle environment."