Making sure that the Soviet Union adheres to an agreement limiting medium-range nuclear missiles would be much more difficult than verification of other arms accords such as SALT II, according to a number of American officials and private analysts. Thus negotiations on a superpower medium-range missile pact could still stall, despite a promising concession made by Soviet leader Mikhail Gorbachev on Saturday.
``We are entering an end game here that's going to be very tricky,'' says Michael Krepon, a verification specialist at the Carnegie Endowment for International Peace.
Past SALT accords have put limits on large weapon launchers easily spotted by spy satellites, such as silo-based intercontinental ballistic missiles and long-range bombers. Medium-range nuclear weapons such as the Soviet SS-20 are generally smaller and based on mobile launchers, making them harder to track from the sky.
In addition, the Reagan administration's negotiating position is that any medium-range pact should limit actual warheads, with the United States allowed 100 warheads, based on its territory, and the Soviet Union allowed an equal number, based in its Asian regions. Warheads, even smaller than missiles, are that much harder to count.
Some sources say the Pentagon is taking a hard line on this point, and that any agreement should have a strict limit of 100 warheads existing in each country, period. Analysts outside the government say such an agreement would be extremely difficult, if not impossible, to verify.
These analysts say subtly different proposals would be easier to enforce, such as one allowing 100 warheads based near launchers, or just a limit on the launchers themselves.
There are indications the US is still flexible on how the warhead limit would be defined. ``The number of spare parts and replacements you would be allowed - these are details that still have to be addressed,'' says an Arms Control and Disarmament Agency (ACDA) official.
However this issue is addressed, the US position has long been that any agreement on midrange nuclear weapons would have to include something radically new in the relations between the superpowers: on-site inspection of each other's weapons facilities.
The US has proposed in previous negotiations on midrange missiles that each side be allowed periodically to send inspectors to production plants and deployment sites for verification purposes. To make them effective, they would have to be surprise visits, not planned, the ACDA official says.
``It doesn't do any good if they have 30 days' notice,'' he says.
The Soviets have made general positive noises about this provision, which is a promising development, US officials say.
But they have never responded to the fine print, as they will have to now that Gorbachev has moved negotiations along by agreeing to delink intermediate arms and limits on the Strategic Defense Initiative.
On-site inspection would cut both ways. There would be Soviet inspectors running around US installations in Europe on short notice, something that some US allies reportedly object to.
At a White House briefing Monday, a senior administration official hedged a bit on European objections, saying ``We haven't seen that we have a significant problem within the alliance yet, on that subject.''
Analysts have long felt that intermediate-range weapons were the most promising area for an agreement between the US and the Soviet Union.
Gorbachev's delinkage proposal is widely seen as removing the highest obstacle to such a pact.
But in diplomacy details can be everything, and analysts warn that arcane details could still cause progress on this issue to come to a crashing halt. The onus for such a collapse could fall on the Reagan administration.
``The US could make the verification issue nonnegotiable if it chooses,'' says James P. Rubin, research director of the Arms Control Association.
Mr. Rubin says he believes such a pact would be verifiable. He points out that the Pentagon now announces with apparent confidence an exact number of deployed Soviet SS-20 missiles.