Do Soviets use weapons the world has banned?

United States officials say they now have ''devastating'' evidence that the Soviet Union is responsible for thousands of deaths due to chemical or biological weapons used in Southeast Asia and Afghanistan.

If true, this means the Soviet Union could be violating international agreements governing the production and use of such weapons. While verifying compliance with such treaties has always been difficult, officials here say, these latest revelations cast a shadow over present and future arms negotiations , including those addressing nuclear weapons.

The subject of ''yellow rain'' and other deadly and debilitating substances has been a controversial one, with earlier US evidence of Soviet chemical weapons use having been sharply questioned. But the US says it can now show ''conclusively'' that 6,000 Laotians, 3,000 Afghans, and 1,000 Cambodians have been killed in hundreds of incidents dating back to 1975.

''There's an awful lot of information that continues to come in every day,'' says a State Department official.

In briefings and congressional testimony, officials today (March 22) are scheduled to release declassified US intelligence reports and analyses that, in their view, buttresses their case. Among these are:

* Blood samples taken from rebel soldiers in Kampuchea (Cambodia) that show evidence of mycotoxins, poisons usually produced by fungi, used by Soviet-backed forces there. The use of such biological weapons is prohibited by an international agreement that the Soviet Union has signed.

* New soil and water samples that, in the government's view, indicate that toxins do not occur naturally in certain areas of Kampuchea. These are the same areas where toxins thought to have come from weapons had earlier been found. Earlier, the US had been unable to provide samples of the natural environment - and that had raised doubts about the validity of US charges that the toxins had been introduced into the area by Soviet-backed Vietnamese forces.

* New reports from Afghanistan confirming the use of chemical weapons by Soviet and Soviet-backed government forces there. For example, there is information from groups of Afghan refugees who say they witnessed the aerial spraying of chemicals. This has been confirmed independently by others subjected to such spraying at the same time and place.

The new intelligence information to be released today includes numbers, dates , and locations of such attacks, including the numbers killed and the details of medical symptoms associated with exposure to toxic chemicals.

''There should be no doubt as to what is going on and who's involved,'' says a key official involved in gathering the new information.

Details of increased use of chemical weapons by the Soviet Union and its allies is coming to nongovernment experts as well.

Thomas Gouttierre, head of the Center for Afghanistan Studies at the University of Nebraska, reports an increasing use of deadly and debilitating chemicals as well as ''phosphorous or flame-type bombs'' in Afghanistan.

In recent months, he says, such weapons have been used to prevent the destruction of strategically important areas like high mountain tunnels or religious shrines, while at the same time killing or incapacitating Afghan rebels.

''I'm most concerned about the indiscriminate bombing of villages at an increased level and the use of these chemical weapons,'' he said.

Mr. Gouttierre is a recognized expert on Afghanistan and has spent 10 years in that country. He has continuous contact with Afghan refugees in Pakistan, and gets more information from letters smuggled out to the more than 250 refugees who have settled in Omaha.

''The information I'm getting is hearsay,'' he admits. ''But it's consistent, and it is varied enough in its source that I am pretty much convinced of its authenticity.''

It is difficult to prove without doubt that such weapons are being used, officials acknowledge. It would be impossible to produce, say, a spent chemical canister with ''Made in Moscow'' stamped on it. But administration officials who have been pressed to offer such proof say they now in effect have ''the smoking gun'' they need to make their case.

Even Sen. David Pryor (D) of Arkansas, who has been leading the fight against increased US chemical weaponry, says: ''I think we have to face the fact that the Soviets may be using them in Southeast Asia and Afghanistan.''

It remains to be seen what actions will follow these latest US intelligence revelations.

A United Nations panel of experts gathered some tentative information in Thailand last November. While it did not endorse the US argument that biological warfare was being conducted with mycotoxins, it said medical symptoms of persons injured in Laos and Kampuchea ''could suggest a possible use of some sort of chemical-warfare agent.''

The UN in December voted to continue its investigation, and the new US report is expected to give impetus to that effort.

The US contends that the Soviet Union already is violating the 1972 Biological Warfare Convention by failing (as the Soviets and other signatories agreed) ''to consult one another and to cooperate in solving any problems which may arise'' regarding the convention's provisions.

The new US charges also no doubt give ammunition to the Reagan administration in its push for increased US chemical-weapon capabilities. The Pentagon wants to boost chemical warfare spending from $532 million this year to $705 million in 1983, including $30 million to produce new binary chemical artillery shells and bombs.

''We have come to the reluctant conclusion that a merely defensive capability is not enough and that we must have a retaliatory capability,'' Assistant Secretary of State Richard Perle told a Senate panel last week. He stressed that this was for the purpose of deterring the Soviets. Special correspondent Edward Girardet reports from Paris:

French doctors whose missions take them into eight Afghan provinces report no first-hand evidence of use of chemical warfare by Soviet and Afghan forces. But there are so many Afghan accounts of chemical warfare that the doctors, among the best sources of information on Afghan events, conclude the use of some kind of chemicals is ''99-percent certain,'' even if the substances are unknown. The doctors have photographs of the blackened faces of victims, which suggests they were victims of napalm or chemicals. It is impossible to know the number of such victims in wartime conditions, and estimates that are bandied about are sometimes the product of imagination.

The use of phosphorous bombs by the Soviet side appears on the rise. When these destructive weapons explode, they spread a sticky burning substance which can reignite up to 24 hours later. An exploded bomb recovered from Afghanistan during a reporting trip was analyzed by the French Ministry of Defense. The weapon was described as a phosphorous-magnesium bomb with an unusually high 4.2 percent concentration of mineral phosphorous. This is slightly different than a conventional phosphorous bomb, according to the French, but is not classified as a chemical bomb. Mycotoxins: the lethal element in 'yellow rain'

Medical experts say the critical lethal agents in ''yellow rain'' are poisonous substances produced by a grain fungus. These are three tricothecene mycotoxins which, scientists say, can cause vomiting, hemorrhaging, and eventual death. The US says it has found mycotoxins in levels up to 20 times that occurring naturally in areas subjected to ''yellow rain.'' Critics, however, have charged that American testing procedures are inadequate.

The manufacture of toxins (the non-living product of living organisms) for military use is forbidden under the 1972 Biological Weapons Convention. But an East German military manual contends that ''toxins are not living substances and in this sense are chemicals.'' The difference in definition is critical; while biological weapons such as toxins may not be produced under existing treaties, there is no such prohibition against the manufacture of chemical weapons.

''They [toxins] thus differ fundamentally from the biological organisms so that they can be included among chemical warfare agents,'' the East German manual states.

The United States is intending to produce new ''binary'' chemical weapons, including a 155-millimeter shell and the ''Bigeye'' bomb.

The chemicals in binaries (which attack the human nervous system) are not new to the US arsenal. The production of these substances is permitted under current international law. And US officials say these weapons are safer because the poisonous chemicals are not mixed until the weapon is actually dropped or fired. US lags behind Soviets in chemical weaponry

Actual amounts are classified, but the Center for Defense Information puts the stockpile of US nerve gas weapons at about 130,000 tons. The total, including military chemicals (mustard gas and two types of nerve gas) that are stored in bulk, is 400,000 tons, according to this estimate.

These include nerve gas projectiles, land mines, spray tanks for aerial delivery, and bombs. The Defense Department is studying the use of multiple rocket launchers and ground-launched cruise missiles as new chemical weapon vehicles, officials testified last September.

The US stopped making new chemical weapons in 1969, and the Pentagon says much of its stockpile has deteriorated.

According to James Wade, deputy undersecretary of defense for research and engineering, ''Today, the Soviet Union has the best equipped and trained forces in the world for conducting sustained operations on a chemical battlefield.''

US intelligence estimates give the Soviet Union a 4-to-1 (and perhaps as much as 10-to-1) advantage in chemical munitions, an 11-to-1 edge in chemical personnel, and a 14-to-1 lead in chemical weapons production facilities.

In secret testimony last week, intelligence officials told a congressional panel that the USSR has at least 6 military institutions (including four-year academies) devoted exclusively to chemical warfare. The Soviet military includes 60,000-100,000 chemical specialists.

By contrast, US soldiers get relatively little training in chemical warfare. Weapons the world wants to outlaw

The 1925 Geneva Protocol prohibited ''the use in war of asphyxiating, poisonous or other gases, and of biological methods of warfare,'' but did not prohibit their manufacture or stockpiling. This treaty was signed by both the Soviet Union and the United States.

Forty-seven years later, the 1972 Biological Warfare Convention treaty (also signed by both countries) banned ''the development, production, and stockpiling of bacteriological and toxin weapons.'' This was an absolute prohibition against biological weapons, but still did not regulate the production and stockpiling of man-made chemical weapons.

Little progress has been made in achieving a complete and verifiable ban on both chemical and biological weapons, however. Such talks between the US and the USSR were begun in 1976, but none have been held since July, 1980. Each side accuses the other of impeding progress, and now the US says it needs a stronger retaliatory chemical capacity in order to force the Soviets to negotiate.