The United States and the Soviet Union appear to have broken the lenghty impasse in strategic arms negotiations during three days of "intensive" talks that ended here today.

The extent of progress the secret discussions made toward actually laying the groundwork for new agreemnts will remain unclear, however, until Saturday, when a join communique is to be issued and Secretary of State Cyrus Vance is to hold a news conference.

After almost eight hours of negotiations with the Soviet delegations headed by Foreign Minister Andrei Gromyko, U.S. officials seems pleased that at least the process of negotiating and bargaining with the Soviets have been restored.

The talks here were the first since March, when the Kremlin rejected the initial proposals of the new Carter administration and plunged already strained Soviet-American relations into a new period of uncertainty.

Returning from the final session here tonight, Vance told reporters only, "The talks have been very useful as far as we are concerned and I believe the Soviets share the view."

Despite the resolute positions with which both sides entered these talks, the key to the apparent progress here has been both countries' willingness to move to some degree from these positions.

U.S. officials have not predicted that these meetings would produce an actual agreement to replace the existing five-year Soviet-American arms pact, which expires Oct. 3.

Rather, the talks were billed as aimed at getting negotiations started again in a more relaxed atmosphere and providing a "framework" from which both sides could work toward more detailed agreements.

Although no details of the tightly held discussions have been revealed, the compromise most widely anticipated involves a modified version of the agreement reached by former President Ford and Soviet Communist Party leader Leonid Brezhnev in Valdivostok in November 1974. Those agreements were never formalized, however.

This modification would involve some slight reductions in permitted levels of nucearl-tipped missiles and bombers in each arsenal and, possibly, some formula to deal with the highly controversial questions of new U.S. cruise missiles and Soviet Backfire bombers.

Such a compromise might also be similar to another deal worked out privately by former Secretary of State Henry Kissinger and Brezhnev in Moscow in January 1976. That also was never consummated.

Those 1976 negotiations represented the last real bit of bargaining between the two superpowers on limiting and perhpas eventually reducing atomic weaponry.

A modified version of the Vladivostok and Moscow discussions would represent a position closer to the one favored in the Kremlin, as opposed to the plan for far more sweeping arms reductions and limits on missile modernization favored by President Carter.

U.S. officials insist, however, that any new agreement contain a commitment by both sides to begin work immediately on a third SALT agreement that would seek those broader objectives.

The first public indication of Soviet movement from the previously firm position came in the first of two sessions today.

State Department spokesman Hoding Carter told reporters that the Soviets had made "a presentation" during a session in which Gromyko did all the talking.

Carter declined to describe this as a "comprehensive proposal," apparently in an effort to avoid even the slighest embarrassment to either party. Both sides came here insisting that they were bringing nothing new. Carter said the Soviets presented "a number of ideas."