Bangkok, Thailand — The liquid drumming on the roofs of their huts sounds like an unexpected cloudburst to the Cambodians and Laotians who report they have come under attack by ''yellow rain.''
This is one way in which the substance known as ''yellow rain'' has been described by victims who later fled to Thailand. Both the Soviet- and Vietnam-backed Lao government and the Vietnam-backed government of Kampuchea (Cambodia) are alleged to have used the weapon to suppress internal rebellions.
United States authorities have concluded that ''yellow rain'' is a new kind of weapon that originates from the Soviet Union and violates a 1972 convention against biological warfare. The US argues in the most compelling of its evidence released Monday by the State Department that the weapon contains a mycotoxin (a toxin produced by a fungus).
But nongovernment scientists have sometimes argued there is still too little firsthand evidence to identify the substance as a new form of mycotoxin weapon, as opposed to one or several more-conventional chemical weapons.
Now there may be an opportunity to get new, more convincing evidence to shed light on the controversy.
Samples of blood and other substances taken from the victims of fighting in Kampuchea two weeks ago have been sent to Washington for examination. About 100 people were affected in that incident: Seven guerrillas of the ousted Khmer Rouge government of Kampuchea died, allegedly after being hit by ''yellow rain'' in a Vietnamese attack near Thailand's eastern border.
Three young soldiers who are still considered very ill may provide the most compelling evidence yet gathered on the use of ''yellow rain.''
According to a US Embassy official in Bangkok, those samples are significant because they were collected so quickly after the attack. In many cases months elapsed between attacks and the arrival of samples at laboratories.
A United Nations investigating team recently in the area took samples of ''yellow rain'' on vegetation and other surfaces, but they were at least six months old, far too old to be of value, according to experts.
After an alleged ''yellow rain'' attack in western Kampuchea last month, 11 guerrillas and civilians became ill and one soldier later died. Samples from the body of the deceased are being analyzed in the United States.
The most recent alleged incident of biological or chemical warfare occurred 10 days ago during Vietnamese attacks on Sokh Sann in the extreme southwest of Kampuchea, 250 miles from Bangkok.
The leader of the anti-Vietnamese Khmer Peoples National Liberation Front (KPNLF), former prime minister Son Sann alleged that ''yellow rain'' and a toxic gas in artillery shells exploded over Sokh Sann on March 10.
People exposed to the substances suffered dizzy spells and the other symptoms , but there had been no deaths.
Samples of the allegedly toxic material were collected before the Vietnamese overran Sokh San March 21.
American and other Western diplomats in Bangkok say there has been wider use of chemical warfare against anti-Vietnamese forces since the beginning of the year. Fresh evidence is also coming in from Laos.
American officials are relieved that other countries have joined in the investigations of ''yellow rain'' and other chemical weapons.
Britain recently collected material from Laos, and it is being examined in laboratories in England. Canada is also investigating.
American intelligence analysts say the Vietnamese are using ''junk'' as well as lethal substances. The ''junk'' is probably a mixture of irritants including tear gas left behind in Vietnam by US forces.
The yellowish powder dropped by a Vietnamese plane over a Thai border village last month may have been ''junk.'' The Thai military first called it ''yellow rain.'' Later an Army spokesman said the powder contained three types of nontoxic chemicals that appeared harmless to humans.
He suggested the powder had been dropped as part of psychological warfare. However officials in Washington and some Thai specialists say that judgment may have been too hasty.
Some of the Thai leadership appear to be playing down the ''yellow rain'' claims, possibly in favor of a softer approach to Vietnam.
But the Thai supreme command declared in the past few days: ''Vietnam has extensively used against Khmer Rouge guerrillas yellow toxic chemicals dropped from planes, fired in artillery shells, and thrown into streams and other water sources.''
Western diplomats, including defense attaches, now generally are convinced that Vietnam and its allies are waging biological-chemical warfare in Kampuchea and Laos.