A team of U.S. chemical warfare experts will tour overseas capitals next month to gather support for creation of an international commission to investigate charges that the Soviet Union or its allies are using poison gas in Laos, Cambodia and Afghanistan.

The idea, officials said yesterday, is not to convince other countries on the spot that communist forces are in fact using such weapons. Rather, it is to make the case that there is sufficient concern and circumstantial evidence to warrant setting up a neutral commission of scientific experts or some other international forum.

There is also an awareness that charges by the United States alone probably would be viewed as propaganda in many parts of the world.

At a joint session of two House Foreign Affairs subcommittees yesterday, Thomas D. Davies of the U.S. Arms Control and Disarmament Agency testified that: "We recognize that in the present international political environment it may be difficult to gain broad support for an investigation.Many countries, including some friendly to us, may suspect that our principal motivation is to embarrass our adversaries; this is not the case."

Others, he said, "may consider our approach hypocritical in view of the U.S. use of riot control agents and herbicides in Vietnam" which are non-lethal.

"Nonetheless," Davies told the panel, "we are determined to press ahead and to try to persuade others of the need for an investigation. We recognize that to be accepted our approach must be based on principle."

Another witness, Harvard University biochemistry professor Matthew Meselson, challenged administration allegations about the extent of poison gas usage, but also called for international investigations.

Noting that reports of chemical warfare in Laos have been heard for years and those involving Afghanistan for four months without hard, confirming evidence, Meselsen said "what we have in the present situation is the worst of both worlds.

"If the unconfirmed allegations of the use of poison gas are false, continued doubt serves only to erode the existing restraints against chemical warfare and to undermine the basis for effective arms control.

"If, on the other hand, the allegations are true, our inability to document them prevents us from having much impact on the actual course of events."

The Geneva protoctol of 1925 prohibits the first use in war of lethal or incapacitating chemical agents. But the protocol provides no mechanism for investigation.

Futhermore, Undersecretary of State Matthew Nimetz pointed out to the panel yesterday that Cambodia, Laos and Afghanistan are not parties to the protocol and that the Soviets adhered to it with a reservation that they would not be bound with regard to states which are not parties.

Aside from approaching allies and key nonaligned governments, Nimetz said the United States was considering raising the issue in the United Nations General Assembly session this fall.

Nimetz said, "We are not in a position either to confirm or disprove conclusively reports of the use of chemical weapons in remote areas where the U.S. government has no presence."

But, in a country-by-country rundown, he said that information "at our disposal" supports the conclusion that Vietnamese and Lao forces have used chemical agents to gain control over dissident tribesmen for several years.

U.S. estimates, based on interviews with refugees, are that about 700-to-1,000 persons may have died and several times that number made ill by chemical agents, he said.

In Cambodia, he said, the evidence is "less substantial" but enough to "warrant serious concern."

In Afghanistan, he said, "we regard it as highly likely that the Soviet invasion forces have used nonlethal chemical agents in their efforts to suppress Afghan resistance" and the chances are "about even" that lethal agents have also been used, based also on refugee reports.

Privately, officials yesterday confirmed the general outline of a Boston Globe report that some Soviet weapons that may be poison gas canisters have been retrieved from the Afghan battlefield and are being studied at the army's Edgewood Arsenal. Other items, including clothing, have also been retrieved.

Yet one official cautioned against putting too much hope into such efforts to gain "hard evidence." He called the chemical warfare field "the most complicated and frustrating" in terms of trying to find traces of short-lived agents.

Inability to perform autopsies, or even to bring bodies out over long distances, or to quickly and properly encase possibly contaminated equipment at the scene, creates monumental problems of proof, he said.

Meselson also challenged public State Department suggestions in March that the outbreak of anthrax in the Soviet city of Sverdlovsk last year was caused by an accident at a Soviet plant producing biological weapons.If true, this could mean Moscow violated a 1975 convention that bans production of such poisons.

Meselson said reports in the West Germany, Israeli and U.S. press about the incident left him "with serious doubts.

He said research of Soviet literature has turned up 100 references to the disease in articles over 15 years, showing that the disease itself was "a major problem" generally for the Soviets. He said about 1 million Russians are inoculted against it each year, which means that the Soviets must have to grow the bacteria in order to produce the vaccine.

He said he was also unconvinced by the Soviet explanation of what happened, but felt it was not all clear that a treaty had been violated.