The administration's "Star Wars" Strategic Defense Initiative (SDI) and the ending of current arms control limits may cause the Soviets to build and deploy more nuclear missiles capable of hitting the United States, two administration reports forecast.

The administration's annual arms control impact statement to Congress repeats the White House position that "reduction of offensive arms will remain the top arms control priority."

But the statement, drafted by the Arms Control and Disarmament Agency (ACDA), said it could be harder to get Moscow's agreement on new constraints on the building of weapons, "particularly if the Soviets calculate that they can counter the SDI with offensive deployments of their own."

Current Geneva negotiations to limit or reduce U.S. and Soviet offensive nuclear forces may have been made "more complex" because of "uncertainties" over SDI, according to the statement released yesterday.

Until now, administration spokesmen have said they hoped the Soviets would agree in Geneva to reduce intercontinental and intermediate-range nuclear weapons while SDI research continued.

"Negotiations of deep reductions in offensive forces" could be "facilitated," ACDA said yesterday, only if both sides were in the process of pursuing "effective defenses."

A Pentagon report released last week also held that a first Soviet response to SDI probably would be "increasing missiles, warheads and penetration aids in an attempt to saturate the defense."

If the current SALT II treaty limits on multi-warhead missiles were scrapped, "the number of Soviet ballistic warheads could increase to at least twice their current levels with only a modest increase in the number of ballistic missile boosters," the report added.

The Pentagon report also said Moscow could attempt to combat SDI by improving its missile force with "warhead deployment techniques . . . and missile-basing schemes that would make them less detectable," along with other "qualitative improvements" over the next 20 years "which would allow them to increase further their offensive strike capabilities."

The Pentagon study concluded that the range of Soviet responses to SDI is so broad that no one could claim to forecast with "great precision" what they would be.

In one early approach, the Pentagon study said, the Soviets "might employ a concerted political and diplomatic effort" to force Washington to either drop SDI or negotiate it away.

The unratified SALT II agreement expires at the end of this year. Although both the United States and the Soviet Union have said they would not undercut the treaty, the administration has not decided whether it will continue to adhere to the pact after the expiration date.

In releasing the ACDA report, Rep. Dante B. Fascell (D-Fla.), chairman of the House Foreign Affairs Committee, said he was disturbed by "what appears to be persistent administration waffling on our commitment to SALT."

The United States could exceed the SALT II limits on multi-warhead missiles by 14 this fall, when the Trident submarine, the U.S.S. Alaska, with 24 missiles, is to go on sea trials.

The report repeats the administration's position that "we have not made any decisions about possible compensation in order to remain within the appropriate limits."

Fascell said he found "this lack of certainty disturbing . . . . I take very seriously recent administration reports on Soviet noncompliance. What we preach we must also practice."