Does USSR use chemical warfare?

The United States is continuing its somewhat lonely campaign to publicize and prove charges that the Soviet Union has used chemical and biological weapons in Afghanistan and Southeast Asia.

New evidence sent to Congress and the United Nations this week indicates that the use of the outlawed weapons against resistance forces in Laos, Kampuchea, and Afghanistan continued as recently as last month, despite Soviet denials.

This raises three important points for the near future. Will the Congress now be more willing to increase this country's chemical weapons capability, as the administration wants? Will the UN be more likely to cut through the international political nettles and confront head-on the issue of chemical and biological weapons use? And will arms control negotiations be hampered by the mounting evidence that the Soviet Union may be violating two existing treaties?

Information gathered by Canadian experts seems to confirm the American findings.

Yet aside from additional public condemnation by Great Britain and Thailand, the world has remained largely silent on the issue.

A UN panel investigating the charges will report to the General Assembly next month. But given the nature of the UN, US officials are not optimistic that a hard-hitting report will be produced.

American officials say their evidence shows ''conclusively'' that at least 6, 000 Laotians, 3,000 Afghans, and 1,000 Kampucheans have been killed in hundreds of incidents dating from 1975. This is based on interviews with victims of ''yellow rain'' attacks, other eyewitness accounts, and scientific data obtained from biomedical samples. Information on chemical and biological weapons use also has come from a Soviet soldier who defected to Afghan rebels and a defecting Laotian pilot.

An earlier UN report did not provide conclusive evidence, but agreed that medical symptoms of people injured in Laos and Kampuchea ''could suggest a possible use of some sort of chemical-warfare agent.''

The response of the USSR and other critics of the charges is that toxins (poisonous chemical substances produced by living organisms) are found naturally in the environment.

Yet Canadian toxicologists earlier this year (based on their own trip to Thailand) concluded that: ''. . . chemical warfare attacks cannot be explained on the basis of naturally occurring diseases . . . it appears that three different types of agents have been employed as warfare agents, one of them being 'Yellow Rain.' ''

In a report last spring, the US State Department did not have proof of the use of such biological weapons in Afghanistan (although it claimed such evidence for Southeast Asia). This week, however, American officials said they have obtained two Soviet gas masks from Afghanistan (one taken from the body of a Russian soldier), both of which show traces of mycotoxins.

Officials also provided new data on the alleged use of chemical weapons in both regions.

The 1925 Geneva Protocol, signed by both the US and the Soviet Union, forbids the use of chemical and biological weapons in war, but not their production. The 1972 Biological and Toxin Weapons Convention outlaws even the production or possession of biological (but not chemical) weapons.

Both countries acknowledge considerable chemical weapons capability, although it is generally accepted that the USSR is far ahead of the US in this area. The US has been seeking a total ban on the development, production, and stockpiling of chemical weapons through the Committee on Disarmament in Geneva. But verification without on-site inspection would be very difficult, and talks between the two superpowers on the issue have been stalled since mid-1980.

At the same time, the Reagan administration wants to add new chemical munitions (which have not been produced in the US since 1969) to the American stockpile. Officials argue that the US must be able to counter in kind the Soviet capability in order to deter any use of chemical weapons.

Congress in its appropriations process will almost certainly agree to fund new equipment to defend against chemical attack. But it is less likely to buy new chemical weapons.

At the UN, meanwhile, a resolution submitted last week would establish a new permanent panel of experts on chemical and biological weapons to which the UN secretary-general could turn in the event of future charges.