IT IS EASY to sympathize with the administration's mounting frustration over the lukewarm response to its charges of Soviet use of biological and chemical weapons. If the Soviet Union is indeed using chemical nerve agents and biochemical toxins in Afghanistan, Laos and Kampuchea, it is not only a flagrant violation of a treaty commitment but also a program of calculated brutality on a broad scale. If it is happening, it is as important as anything else on the international scene, and deserves far more attention than it has been getting.

Precisely because the charges are so important, however, one is forced to ask why more and better evidence to support them has not been marshalled. Opinions on the strength of the case against the Soviet Union, including those in this space, have swung back and forth as evidence has been released, and errors and gaps in the evidence later discovered. Consider Undersecretary Stoessel's announcement this week that the Soviet Union is waging chemical war in Afghanistan. It was the most direct charge to date by the American government of Soviet violation of the 1925 Geneva Convention prohibiting chemical warfare. But where was the evidence?

Mr. Stoessel's statement, a single paragraph long, said only that Afghan military defectors had provided information on types of chemical agents and where and when they were used, and that this "generally corresponds" with refugee reports and records of known military engagements. Yet the statement claimed that there have been 3,042 deaths from chemical attacks. Even if that figure is only a lower limit, its precision, derived as it is from such crude sources, can only subtract from the credibility of the claim. No physical evidence--weapons shells, photographs, chemical samples--has been found. Asked to provide more information, the State Department said it hopes to issue a declassified report, perhaps within one week.

This sort of thing has been the rule, not the exception. The first announcement of physical evidence of the use of "yellow rain" in Southeast Asia was made on the basis of chemical analysis of a single leaf sample, with no scientific controls. The identification of the source of the toxin included the assertion that the organisms "are not native to warm climates, i.e. Southeast Asia" (emphasis in the original). A few weeks later an independent expert pointed out that natural outbreaks have occurred in India.

In short, if the administration finds the domestic skepticism and international indifference to its charges to be troubling, it has no one but itself to blame. Convincing evidence need not be rigorous enough to meet the standards of a court of law or a scientific journal. These are, after all, active war zones in remote and devastated countries, and collecting reliable proof of the use of these sorts of weapons is a dangerous and technically sophisticated business.

But after years of charges and reportedly "thousands" of refugee reports, it is puzzling--and to some, suspicious--that the government has been unable to document the charges more conclusively. Granted, the evidence is difficult to collect. But with reasonable effort over a period of many years, it should not be that difficult.