Six Soviet diplomats and military experts this week are visiting one of the most sensitive defense facilities in the United States - the Army's chemical weapons depot at Tooele, Utah. The tour is partly an exchange for one given to US officials at a Soviet chemical base last month. For weeks, senior Army officers have been shuttling between Utah and the Pentagon, working on the agenda. Some details had to be approved by the secretary of defense himself.
The exchanges are visible evidence of the progress in negotiations to ban chemical arms. After years of bickering, recent moves by both the US and USSR have infused the multilateral talks with new vitality.
But US government officials caution that an actual treaty is still a distant goal. They say the chemical weapons talks - which involve 40 nations - are perhaps the most complex in the history of arms control.
``The Soviets are sounding more optimistic, but they either haven't thought through big problems that remain or are deliberately raising expectations,'' says a State Department official with knowledge of the talks.
Among the hurdles still to be crossed in the negotiations, which are being held as part of the United Nations-sponsored Geneva Conference on Disarmament:
Verification. Equipment used to produce nerve agent and other chemical weapons is similar to that used for pesticides and other commercial products and is easily hidden. Checking to make sure other nations are not cheating on a ban would be ``a nightmare,'' according to a well-placed Defense Department official.
The US has pressed for on-site surprise visits by inspectors as a chemical ban verification method. The Soviets resisted this measure until last August, when Soviet Foreign Minister Eduard Shevardnadze said his country could accept it in principle.
This move was a large step forward, especially against the background of a treaty on medium-range nuclear weapons that also contains on-site verification provisions. But details of visits - what types of facilities would be covered, for instance - are still a major chemical negotiating problem.
Visiting rights would be a sensitive subject for the US as well as the USSR, especially considering all the countries involved. ``Would this mean Libya could walk in and demand to check our nuclear weapons plants?'' the State Department official asks.
Multilateral problems. Other nations involved in the talks are, for various reasons, sounding hesitant about proceeding with a treaty.
India in particular, as well as some other nations, is reluctant to accept the intrusiveness of on-site verification. West German negotiators have indicated that they are concerned about damage a treaty might inflict on their commercial chemical industry.
A treaty's usefulness could be damaged by the fact that Iraq and some other nations thought to possess chemical stocks are not even joining in the negotiations.
Destruction of stocks. The US and the Soviet Union are likely to tangle over who gets rid of what type of chemical weapon first.
Late this year, the US is scheduled to resume production of chemical weapons for the first time since the late 1960s. The Soviets have said this resumption could hurt negotiating progress, and they are likely to demand that any treaty call for destruction of these new stocks of so-called ``binary'' weapons first. US officials, who claim that the threat of these new weapons is part of the reason that the Soviets have made concessions, say they want to destroy binary weapons last.
Between 1977 and 1980 the US and the Soviet Union had bilateral talks on a chemical weapons ban. When these stalled, the US supported a shift of forum to the multination disarmament conference.
Progress has been slow, but real. With a US draft treaty submitted in 1984 and recent Soviet concessions ``the door is now open'' for an eventual pact, the State Department official says.