Exploratory Soviet-American talks could start "soon" to reopen strategic arms limitation negotiations, Sen. Charles Percy (R-Ill.) said today after three days of wide-ranging Kremlin talks marked by serious differences.

"I would be surprised if both sides would not sit down and agree to talk about arms control leading to possible negotiations for an agreement. . . . Talks could begin soon," Percy said at a press conference at the residence of U.S. Ambassador Thomas J. Watson Jr.

But there were strong indications the process of defining the deep differences over the weapons issue, which the Kremlin says is the foundation of bilateral detente, may be much harder to achieve than Percy was willing to detail.

Of his three-hour meeting today with Soviet Foreign Minister Andrei Gromyko, Percy declared privately, "I was shocked to find even on the third day that the discussion began on the issue of why the Senate had not ratified SALT II." Percy said he told Gromoyko "he must realize my visit is worthless if he does not understand the treaty is dead as a doornail."

Percy, who is to become chairman of the Foreign Relations Committee when the Republicans take control of the Senate in January, had already bluntly delivered that message -- unpleasant to Kremlin ears -- to President Leonid Brezhnev and Defense Minister Dmitri Ustinov in talks Wednesday and Thursday.

Percy said he now thinks the Kremlin leaders "understand" that President-elect Ronald Reagan has no intention of backing the treaty that Brezhnev and President Carter signed with an exchange of kisses and bear hugs in Vienna in 1979. But, Percy asserted, the Kremlin still does not accept the position of Reagan and the new Republican-dominated Senate.

Brezhnev, Gromyko, and other Politburo leaders have repeatedly declared that Moscow will "never" agree to reopen the treaty, insisting that it must be ratified as signed.

Behind the scenes, however, the Soviets have been discreetly feeling out the new administration's aims and views. This effort came into the open with the visit of Percy, who has had remarkable access to the highest leaders here during what was intended months ago as a private visit to his longtime friend, Watson, and became an official mission heralded by the Soviet media.

Despite the local press attention, taken as a sign of Kremlin eagerness to set a positive tone in relations as Reagan comes to power, the formal sessions have achieved no discernible breakthroughs in the tense and troubled Moscow-Washington relationship.

Percy's account of how arms talks might be reopened showed that it could take much time to narrow differences. "There could be informal discussions without a formal agenda, like the format we had, and we could just talk and get to know each other," he said. "That is the process that has to begin.

"Once they are reaching a reasonable agreement on certain principles, I think then it is time to establish a formalized agenda and go into formal negotiation. I think they will agree very quickly on the need for an arms control agreement that will be meaningful, effective, and hopefully have both ceilings and reductions" in weapons systems and warheads.

If there were Soviet willingness to start, Percy said, he, as foreign relations chairman, could try to improve relations by holding hearings on two Soviet-U.S. treaties on underground and peaceful nuclear explosions. The treaties have been stalled in the committee since being signed in 1974 and 1976.

Percy said the Soviets made no mention of trying into these bilateral strategic arms talks the additional issue of limiting theater nuclear forces.

As before, Percy would not disclose the substance of Soviet replies or detail their positions, saying he must brief the Regan camp first.