A group of U.N. experts investigating American charges that the Soviet Union has used outlawed chemical weapons in Afghanistan and Indochina, issued a final report today concluding that there is "circumstantial evidence suggestive of the possible use of some sort of toxic chemical substance in some instances."

The report confirmed that victims had been exposed "to mycotoxins of the trichothecene type," commonly known as "yellow rain."

But the four experts from Egypt, Kenya, Peru and the Philippines did not conclude that the allegations had been proved, because the toxins found in the samples they collected "could be attributed to natural causes."

The results were expected but disappointing to American officials, who had presented their own evidence, in a State Department report issued a week ago.

"I think we conclusively proved the toxins could not be natural," said Gary Crocker, chief of the American monitoring team.

Today's U.N. report contained more tangible evidence and leaned more toward the American argument than the report issued by the same four experts one year ago. The evidence was gathered on trips to Thailand and Pakistan, where refugees from Cambodia, Laos and Afghanistan were interviewed and samples were collected for analysis.

Crocker said the U.N. experts "tried hard to do a good job," and "were all sincere people." He maintained that they were hampered by limits on where they could go, by the presence of a Bulgarian national on the support staff assigned by the United Nations and by delays in laboratory analysis of blood samples that "degrade quickly."

As a result, American officials have decided against pressing for any future report by the present U.N. team. Instead, the United States is backing a French resolution, to be taken up by the General Assembly on Wednesday, that asks the secretary general to develop a permanent U.N. capacity to dispatch experts to the scene of an alleged chemical warfare attack on an emergency basis and to line up laboratories in various countries that are capable of analyzing toxin samples.

With such a capability, Crocker said, the United Nations could react quickly to new reports of chemical weapons attacks. He said that in addition to the continuing use of toxins against insurgents in Afghanistan and Indochina -- some as recent as October -- there have been reports of the use of similar chemical agents by Ethiopia, another Soviet ally, against rebel groups whose survivors have fled to the Sudan.

No other government has echoed the American allegations, made here and in Washington, that Moscow has violated its obligations under the Geneva Protocol of 1925 on chemical warfare and the biological warfare treaty of 1972.

The major factor in the reluctance of other nations to criticize Moscow, Crocker said, appears to be the political implications of any proof that it does not live up to its treaty commitments.

"It is perceived that if we prove this charge, it will somehow do away with arms control. We disagree. We think it shows that we need better treaties, with verification and control measures in them," Crocker said.

The Soviet Union has dismissed the charges as lies, motivated by the anti-Soviet views of the Reagan administration. Soviet officials have maintained that any evidence of toxins can be attributed to chemicals occurring in nature. The United States has countered with assertions that such toxins do not occur naturally in either Afghanistan or Indochina.