The Soviet Union is building two or three new bases for SS20 intermediate-range nuclear missiles in central Asia, U.S. officials say.

Each base usually contains nine of the three-warhead missiles that can reach targets up to 3,000 miles distant. The SS20 missiles are at the heart of the deadlocked arms control talks that the two superpowers are holding in Geneva.

U.S. officials now credit the Soviets with 351 operational SS20s, of which 243 are based in the European portions of the U.S.S.R. west of the Ural Mountains and presumably aimed at NATO targets in western Europe.

The other 108 are based east of those mountains and presumably are targeted on China, Japan and South Korea.

The new bases could add 18 to 27 missiles and 54 to 81 warheads to Soviet forces in Asia.

Officials describe these new bases, however, as "borderline," in that, although they are in central Asia, they are reportedly in locations from which the missiles could still reach some of the countries on NATO's northern and southern flanks, such as Norway, Greece and Turkey.

There is also a possibility that they could reach western Europe, officials said, because there is some uncertainty about the SS20's range.

Either way, State Department officials said new bases could further complicate the Intermediate-range Nuclear Force (INF) talks scheduled to resume on May 17 in Geneva.

The Soviet missiles are mobile, which means they could be moved. This is why the United States is insisting on limiting these weapons in Europe and Asia.

The United States is scheduled to begin deploying the first of 572 new Pershing II and cruise missiles in Europe in December to offset Soviet forces unless an agreement on some equal but lower level of forces can be achieved before then.

The State Department's disclosure that "new SS20 bases have been begun in central Asia" came in response to an article on the op-ed page of The Washington Post last Tuesday by Raymond L. Garthoff, a leading expert on Soviet issues and a member of the U.S. team that negotiated the 1972 strategic arms limitation agreement with Moscow.

Garthoff was writing about a dispute between Soviet leader Yuri V. Andropov and President Reagan over a supposed Soviet moratorium on future SS20 deployments. Reagan had accused the Soviets of violating a promise to stop deployment. In a response March 26, Andropov accused Reagan of telling "an untruth" on the subject.

In March, 1982, Soviet President Leonid I. Brezhnev announced a decision "to introduce, unilaterally, a moratorium on the deployment of medium-range nuclear armaments in the European part of the U.S.S.R." The announcement was of public relations importance in western Europe, where Moscow seeks to bolster opposition to the December U.S. missile deployments.

Garthoff said that Brezhnev's exact language had been lost in the rhetoric since on both sides. He noted that Brezhnev only promised not to deploy more medium-range nuclear missiles in the European parts of the Soviet Union. He also said there was precedent for construing a future moratorium to allow completion of sites under construction.

Moscow had 207 SS20s in eastern Europe and 36 under construction there at the time Brezhnev made his statement. That adds up to 243, the same as now, Garthoff noted, implying that no promise had been broken.

The State Department seemed on one hand to acknowledge this; it said "the United States and NATO have not charged the Soviets with beginning new bases in the European U.S.S.R."

But the department said the Soviets nevertheless have been "very misleading" and that "the moratorium announcements made by the Soviets last year clearly indicated that the Soviets were halting all activities associated with additional SS20 deployments oriented against Europe."

To support this, the State Department pointed to a May 18, 1982, Brezhnev statement that Garthoff did not mention.

Responding to western criticism, Brezhnev said: "Another question that is being posed is whether the decision adopted by us also envisages a unilateral freeze on preparations for the ultimate deployment of missiles. Yes, it does envisage this, including an end to the construction of launching positions."

State Department officials said this clearly gives the impression to the "man in the street" that Moscow was also halting construction of bases then under way.

This construction did not stop, the officials said in defense of the Reagan position, nor have the deployments in Asia stopped.