The Thai military and a number of foreign defense attaches in Bangkok are convinced that Vietnam is using toxic chemicals against guerrilla forces opposed to them in Cambodia and Laos.
The evidence that cyanide is being used in some form appears conclusive, according to some of the attaches with knowledge of tests carried out on water and vegetation from border areas of Cambodia and Laos.
The Thai Army said recently it had identified cyanide in water from wells and streams in western Cambodia near the Thai border.
American officers also said they had evidence that Vietnamese forces were using cyanide, although the evidence suggested it had been spread by hand rather than used in weapons. Some samples had been sent back to Washington for analysis.
Last week a deserter from the Vietnamese Army in Cambodia, Capt. Nguyen Quarn , a veteran with 16 years service, said that his artillery unit had fired Soviet shells containing what he called "CH" gas from 130-mm. Soviet guns on numerous occasions. he said the gas killed civilians and guerrillas of the former Khmer Rouge government.
Captain Quarn said his unit had fired 700 rounds of the gas shells in one attack and 400 in another. He said the gas vapor generally killed people when it fell directly on them but only temporarily disabled them if it missed them by 20 yeards or more. He and his own men hd used gas masks during the attacks.
He sid the Vietnamese Army used the gas shells from the early stages of its invasion of Cambodia in December 1978 and was still using them in what he called "hard situations."
Captain Quarn's designation of the gas as "CH" is thought by military experts here to mean hydrogen cyanide, a compound known to be among the Soviet Union's massive armory of chemical weapons. It causes "rapid respiratory failure," according to experts.
He said all the Army's weapons in Cambodia were from the Soviet Union. He had seen large numbers of Soviet advisers who were attached to every regimental and divisional headquarters of the Vietnamese Army.
They gave military and political advice, he said, but did not participate in fighting. He described relations between Russians and Vietnamese as "normal."
Red Cross and other relief officials are reluctant to speak openly about such an emotional subject as chemical warfare, but privately most admit they are convinced the Vietnamese are using poison substance in some form or other against their enemies in Cambodia and Laos.
In March an International Red Cross official said 65 Cambodians had been treated in four Red Cross hospitals on the Thai border for a type of poisoning. There was at least one death.
The official said he did not intend to say anything about the poison substance because to identify it was to make allegations.
The Thai Army, however, after testing water and vegetation from the affected area, said it had found cyanide in the samples.
About the same time, Cambodian guerrillas claimed that 17 of their people had been killed and 200 wounded in a gas shell at tack by Vietnamese forces.