As US, Soviets talk, chemical weapons 'club' grows; Kremlin's acceptance of inspection improves prospects for pact; 15-20 nations have chemical-warfare capability

While the United States and the Soviet Union continue their public political pas de deux on nuclear arms and weapons in space, the two superpowers are engaged in quieter behind-the-scenes discussions on limiting chemical weapons.

After a three-year hiatus in such talks, Moscow - in a major step for the Kremlin - has0agreed to international inspections on Soviet soil to verify the destruction of chemical stockpiles and production facilities. And the Reagan administration - defeated in its effort to win congressional funding for new chemical bombs and artillery shells - has advanced a treaty proposal to ban such weapons.

Still, the likelihood of reaching a comprehensive, multinational agreement in this most difficult area of arms control is far from assured, say US officials. And the chances that chemical weapons will be used are increasing steadily.

''All too often in the past, the nuclear issue has so overshadowed as to drive out concerns on chemical weapons,'' Kenneth Adelman, director of the US Arms Control and Disarmament Agency (ACDA), told a Senate hearing. ''I personally put the threat of a nuclear war low, very low. I . . . put the increasing use of chemical weapons around the world high.''

Until recently, US intelligence agencies had assumed that about five countries possessed chemical warfare capabilities. But this estimate now has increased to about 15 countries. Experts at a recent conference in Belgium put the figure at 20.

Proliferation to the third world - as evidenced by Iraq's use of mustard gas against Iranian soldiers - makes the possibility of terrorist use especially worrisome. Such proliferation, says chemical weapons expert Brad Roberts of the Georgetown University Center for Strategic and International Studies, ''is proceeding apace as nascent capabilities spread, technologies and perceptions change, and the will to use them becomes easier to muster.

''They have two things that make them particularly appealing (to terrorists), '' says Mr. Roberts, author of a recent Library of Congress report on chemical weapons. ''One is that they are low-tech. They are easy to use. They have another characteristic that is desirable, and that is that they have a very high shock value. . . . They can be a poor man's nuclear bomb in certain contexts.''

The US and several European countries have restricted the shipment abroad of materials that could be used to make chemical weapons. But experts say it remains relatively easy to manufacture such weapons using homegrown supplies.

''Production is easy to conceal,'' Defense Department official Douglas Feith told lawmakers. ''As we have seen in Iraq, insecticide factories can serve as cover. Insecticide, after all, is nerve gas for insects.

''While export-control successes are possible in specific instances if international cooperation is forthcoming, . . . no controls can reasonably be expected to block a determined country for a long time from acquiring chemical weapons,'' he added. ''The problem is not the inadequacy of our laws or their enforcement; it is the nature of the chemicals in question.''

Some 120 countries, including Iran and Iraq as well as the United States and the Soviet Union, signed earlier international treaties banning the use of chemical and biological weapons. But those treaties relied on the signatories' good will for compliance and have no strict provisions for international verification.

In its draft treaty, presented by Vice-President George Bush before the Conference on Disarmament at Geneva in April, the US says countries should have an ''open invitation'' to demand on-site inspection ''anywhere, anytime'' they suspect violation by another country. Critics say this could diminish, if not kill, the chances for an agreement.

''There are other countries besides the Soviet Union which will have difficulty in accepting that kind of provision,'' says Charles Floweree, former US representative to the Conference on Disarmament. Moscow has offered to allow inspection of ''declared'' chemical weapons sites, but objects to any kind of blanket inspection provision.

US intelligence agencies and the military services also are wary of the prospect of Soviet agents snooping around US facilities. But the US position, as stated by Vice-President Bush, is that ''we . . . are willing to pay the price of such openness.''

US chemical industries are described by government officials as ''cooperative'' in trying to stem chemical weapons proliferation even though, as ACDA director Adelman says, on-site inspection could mean ''a tremendous inconvenience and disruption for them.''

The Reagan administration has been accused of dragging its feet on arms control. And US officials are not sanguine that the kind of international agreement on chemical weapons they seek will be easily won.

But there is a strong feeling within the administration, Adelman says, that ''only a complete ban can guarantee that a country at war will not, in an hour of desperation, grab for the most terrifying weapon at its disposal.''