Pentagon weighs Soviet arms shift

The latest Soviet offer on limiting strategic nuclear weapons is being viewed by American officials with extreme caution, not only because of its ramifications for the superpowers, but for domestic political reasons as well.

Within the last month, United States and Soviet negotiators at the strategic arms reduction talks in Geneva have shifted positions on limiting intercontinental ballistic missiles (ICBMs) and other long-range nuclear weapons. The latest shift came from Moscow, which now agrees to lower its proposed limits on multiple-warhead missiles.

''It's movement in the right direction,'' said Richard N. Perle, assistant secretary of defense for international security policy and one of the most forceful players on the administration's strategic arms team. ''But we've got a long way to go, and there are other elements in the Soviet position that pose some real difficulties for us.''

Such measured responses are aimed not only at Moscow but at the US Congress, where lawmakers are looking for administration flexibility in arms control before approving such new strategic weapons as the MX missile and B-1 bomber. The Republican-dominated Senate this week overwhelmingly backed a resolution that urges President Reagan to meet with Soviet President Yuri Andropov and seek ''mutual, equitable, and verifiable reductions in nuclear arms.''

US and Soviet arms control offers appear to move the two countries closer to agreement on limiting the most threatening and destabilizing systems. But the offers also illustrate just how far there is to go in reaching a new accord.

The Soviets' offer to limit missiles and bombers to about 1,100 would allow them to keep their largest and most accurate ICBMs - the SS-18s and SS-19s. The Soviet ICBMs are the weapons American officials consider capable of a first-strike, and they want to see their numbers reduced.

''The levels they have in mind, while below those of SALT II (the existing unratified agreement both sides say they are respecting), are nevertheless high and would leave the Soviets in our judgment with significant advantages,'' Mr. Perle said. ''We believe that a useful and stabilizing agreement can be reached on the basis of substantially lower levels of weapons and, in particular, lower levels of the weapons that are most threatening, that is, accurate, high-yield ICBM warheads.''

Ironically, the administration raised its goal for allowable long-range missiles from 850 to about 1,200 to placate arms control advocates. Experts believe this will encourage the development of smaller, single-warhead missiles that would increase strategic stability and reduce the likelihood of nuclear war.

But within this raised figure, the administration still would limit the number of warheads based on land and thus require the Soviet Union to dismantle most of its SS-18s and SS-19s. This would be a considerable concession for Moscow, which has placed about three-fourths of its strategic warheads on land-based missiles.

On Capitol Hill, the administration is caught between conflicting forces. Advocates of a new pact with the Soviet Union want to make sure the administration is fully following the recommendations of the presidential commission headed by former Air Force Lt. Gen. Brent Scowcroft. These include deploying some MX missiles in existing silos, shifting arms control emphasis from launchers to warheads, and developing the new single-warhead missile.

Administration officials insist they are fully committed to the Scowcroft recommendations, and thus hesitate to reject out of hand any Soviet movement at START talks in Geneva. They also say they are, in Perle's words, ''working very hard'' to incorporate the so-called build-down concept into arms control. This notion, urged by a growing number of senators, would require the dismantling of a certain number of existing nuclear weapons for every new weapon deployed.

On the other hand, conservative lawmakers are urging the President to spotlight alleged Soviet violations of SALT II and other treaties in the testing and deployment of nuclear weapons. National Security Adviser William Clark is heading an interagency group that is soon to report on this particularly touchy subject.

Administration officials believe there may be violations, but for the most part blame this on loopholes and ambiguities in existing treaties. This is one reason President Reagan called SALT II ''fatally flawed.'' But they also acknowledge an ''intrinsic bias'' against finding fault with Moscow on this issue. Said one official: ''It is as inconvenient for us as it is for them . . . maybe more so.''