Secretary of State George P. Shultz said yesterday the United States has conclusive proof, including two captured Soviet gas masks contaminated with toxins, that the Soviets are using chemical warfare in Afghanistan.

The gas masks are the first physical evidence to support claims that "yellow rain" toxin is being used in Afghanistan, although guerrilla fighters and journalists have reported incidents of chemical warfare there for several years.

In half a dozen previous reports, the United States has attempted to document the use of chemical and toxin warfare by the Soviets and their allies in Asia, chiefly in Kampuchea, formerly known as Cambodia, and Laos. The evidence has included samples of the "yellow rain" toxin itself from areas of alleged attacks, as well as samples of toxin-contaminated blood, tissue and urine from 16 victims. The samples were shipped to the United States for analysis.

In the new report, Shultz once again accused the Soviets and their allies of violating both the 1925 Geneva Protocol on chemical warfare and the 1972 Biological and Toxin Weapons convention. He called on other nations to speak out against the use of chemical warfare. State Department spokesmen say chemical attacks in Asia have been reported within the past six weeks.

The report delivered to the Congress and the United Nations yesterday said, "For the first time we have obtained convincing evidence of the use of mycotoxins fungal poisons by Soviet forces through analyses of two contaminated Soviet gas masks acquired from Afghanistan."

Officials said that the two masks were standard-issue Soviet-manufactured gas masks, equipped with the latest breathing canister made by the Soviets.

State Department intelligence officer Gary Crocker said that one of the two masks was torn from the head of a dead Soviet soldier somewhere in Afghanistan in December, 1981. The other mask -- which was on display for the press yesterday -- was obtained in Kabul, the capital of Afghanistan, in September, 1981, according to the State Department report.

Crocker said he could not elaborate on how the masks were obtained, except to say that both were obtained as a result of U.S. efforts to get evidence of chemical warfare in Afghanistan.

Both masks were swabbed with solvents which then were analyzed by two laboratories for traces of the "yellow rain," so called because it is often reported by refugees to be sprayed in a yellow cloud of gas from an airplane or a rocket.

The mask obtained in Kabul had the yellow rain toxin called T2, a variety of poison that is a member of the mycotoxin family called trichothecenes, a plant-destroying fungus. Toxins are poisonous substances secreted by certain organisms. Various kinds are alleged to have been used by the Soviets in their effort to subdue resistance forces in Afghanistan and to have been supplied by the Soviets to their Vietnamese and Laotian allies.

Small quantities of the trichothecenes, which have historically been found in agricultural outbreaks of fungus, can cause vomiting, nausea, a severe skin blistering, and sometimes death.

The second mask had T2 toxin as well as several other varieties of trichothecene toxins, according to lab tests conducted by two separate laboratories, including that of Chester Mirocha of the University of Minnesota, an expert on such toxins.

Further evidence from Southeast Asia collected in 1982 was also presented in the report, which stated that "To date, biomedical samples (blood, urine, and/or tissue) from 33 alleged victims have been screened. Specimens from 16 of these individuals show the presence of trichothecene mycotoxins. In addition, six environmental samples from alleged attack sites have been analyzed by Dr. Mirocha. Five show the presence of unusually high concentrations and combinations of trichothecene mycotoxins."

The State Department has said in previous reports that chemical attacks have killed 10,000 persons in the three nations, but "we believe there are many more" that could not be verified from intelligence sources, said Crocker.

State Department spokesmen said that the United States has had some difficulty in convincing other nations and a United Nations commission on the subject to accept their accumulating body of evidence on chemical warfare in Asia.

All but three nations -- Canada, Britian and Thailand -- shied away from the subject, Crocker said, chiefly for political reasons. Skepticism about the evidence itself and whether it could be faked has faded within the past year, chiefly because of the poisons found in the blood and tissues of victims, something that would be extremely difficult to fake.

The United Nations is scheduled to issue its own report within weeks. Officials who said they have seen a draft copy said it takes note of the samples from the United States and other sources, but draws no conclusion about validity of the evidence.

Critics have said that the rules under which the commission is operating are so restrictive that unless the commission collects and analyzes evidence immediately after an attack, no verdict could be reached. The commission has been denied access to the war zones in Kampuchea and Laos, making any such evidence-gathering impossible.