WASHINGTON — In what would be a major breakthrough, Soviet disarmament negotiators appear to have moved far toward accepting a U.S. proposal to put American inspectors at one or more missile production facilities in the Soviet Union to help verify compliance with a ban on ground-launched medium-range nuclear missiles, The Times learned Wednesday.
A comparable U.S. missile production plant or plants, probably those in Southern California that make cruise missiles, would presumably be offered for Soviet inspectors to patrol, U.S. officials said. The agreement would extend over a period of perhaps 10 years.
A State Department official said the Soviets have accepted the U.S. position, but a White House official said the matter is "still under discussion and not nailed down." An accord would help clear the way for President Reagan and Soviet leader Mikhail S. Gorbachev to sign a ban on the medium-range nuclear missiles during their meeting here Dec. 7 to 10.
Although the State Department and White House officials offered slightly different descriptions of how the inspection would be conducted, both said inspectors from the other side would be permanently stationed at entrances to missile plants to check material entering and leaving.
The prospective treaty will also provide for a specified number of sudden visits by inspectors at other locations to watch for cheating, the officials said.
The Soviet move toward permanent monitoring of production facilities came a few days ago during negotiations in Geneva, apparently during talks between U.S. Ambassador Max M. Kampelman and Soviet First Deputy Foreign Minister Yuli M. Vorontsov.
Kampelman and Vorontsov, during a series of meetings that ended Tuesday, also appeared to resolve several other outstanding issues. While some still remain to be settled, none are serious enough to hold up completion of the agreement, the officials said.
Among the new developments:
--The Soviets have given up their demand for a clause in the treaty committing each side to follow-on negotiations to reduce other nuclear arms. The United States and other North Atlantic Treaty Organization nations fear that early new talks would create great public pressure in Western Europe for the elimination of all nuclear weapons there, leaving the Soviets with a commanding superiority in conventional forces.
--The Soviets have also indicated a willingness to compromise their demand for a "non-circumventing" clause. Such a provision would ostensibly aim at preventing the deployment of new nuclear weapons outside the intermediate range (300 to 3,000 miles), but it could also prevent modernization of existing weapons, according to U.S. officials.
The United States proposed instead a clause that would bar activities "conflicting" with the treaty, and the Soviets have now accepted a formulation close to the U.S. position, officials said.
Five Inspections OKd
--In addition to continuous monitoring of declared missile production facilities, the two sides have agreed on five inspections of suspect sites per year for the three years during which the missiles are to be destroyed and their warheads dismantled. They continue to disagree over the number of annual inspections of suspect facilities for the next 10 years, but one official called the difference "not a big problem."
--After repeated delays, the Soviets have now provided information on the number and location of each type of missile to be eliminated. That information was considered crucial to resolve before Reagan and Gorbachev meet here.
But far more significant was Soviet movement on the on-site inspection proposal, both in itself and because of the precedent it will set for verifying compliance with any future agreements reducing intercontinental weapons. The superpowers are now deep into negotiations to cut their offensive strategic arsenals in half.
The U.S. scheme, in which inspectors would effectively surround a plant and check everything that comes out, was proposed several years ago when it appeared that each side would retain 100 mid-range warheads on its territory. Careful inspection was considered necessary to ensure that neither side produced and deployed more than the permitted number.
Subsequently, Gorbachev agreed to ban all missiles after U.S. officials argued that verification measures to police such a treaty could be less intrusive.
Both Cruise Types
However, the United States found that Soviets' medium-range SS-20 ballistic missile, which is to be banned, has some disturbing similarities to the intercontinental SS-25, which is not covered by the prospective agreement. Because of those similarities, U.S. negotiators pressed for on-site inspection of SS-25 production facilities.
The Soviets countered with a demand for on-site inspection of U.S. facilities that produce both ground-launched and sea-launched cruise missiles. Ground-launched cruise missiles are to be banned, but sea-launched cruise missiles are not.
Now the Soviets have apparently accepted some form of permanent monitoring of the SS-25 production plant or plants, and the United States has done likewise for its cruise missile facilities.
U.S. officials expect such monitoring to begin after the three-year period during which the medium-range missiles are to be destroyed, and to continue for 10 years.