The Soviet Union has made a new move in the bargaining over a strategic arms treaty by suggesting to senior American officials in Geneva a possible limit of 5,100 intercontinental ballistic missile nuclear warheads in each side's arsenal, U.S. officials said yesterday.

The Soviet suggestion was characterized by a Reagan administration official as "a step forward" in the negotiations but still not acceptable to the United States. The strategic arms negotiations, which are the top priority of President Reagan and Soviet leader Mikhail Gorbachev, are expected to be the centerpiece of their Dec. 8-10 summit here.

Marshal Sergei Akhromeyev, chief of staff of the Soviet armed forces, presented the suggestion as a possibility under consideration but not as a formal proposal in discussions this week with senior officials accompanying Secretary of State George P. Shultz, according to the sources. Akhromeyev's willingness to discuss it, even on a conversational basis, was taken as a sign that Gorbachev is ready to make a new Soviet move on the difficult nuclear "sublimits" issue at the meetings here.

Akhromeyev's hints evidently were the basis for Shultz's statement in a news conference at Geneva late Tuesday that "we made some progress, no question about it" in the field of strategic arms while completing work on the intermediate-range nuclear forces (INF) treaty to be signed at the summit.

In Santa Barbara, Calif., yesterday a senior administration official appeared to be referring to the same development when he told White House reporters without elaboration that "based on what we discussed with the Soviets in Geneva, the two leaders, I think, can make some progress with respect to the sublimits issue" at the summit. "We can expect some useful discussion and movement on this sublimits question." {Details, Page A19.}

A ballistic missile limit of 5,100 warheads would require the United States to eliminate about 35 percent of its 7,800 strategic ballistic missile warheads, of which about 2,200 are on land-based missiles and about 5,600 are on submarine-based missiles. The same limit would require the Soviet Union to eliminate about 40 percent of its estimated 8,400 strategic missile warheads, of which 6,400 are land-based and about 2,000 are submarine-based.

The crucial and unsolved question for the U.S. side is finding a way to limit sharply the number of Soviet land-based missiles, which have the greatest speed, range, explosive power and accuracy and thus are considered by the United States as the most destabilizing weapons.

The United States is reluctant to accept sharp limits on submarine-launched warheads, which are more survivable and less useful in attacking missile silos of the other side, and where the United States has an advantage.

The U.S. problem with the Soviet idea of a 5,100-warhead limit is that, as discussed by Akhromeyev, it would allow either side to structure its forces as it wished within that overall total. The United States has proposed a limit of 4,800 ballistic missile warheads on each side, but with the provision that no more than 3,300 could be land based. This would require a cut of almost half in Soviet land-based missile forces but leave the United States free to maintain most of its submarine-based force.

In Moscow on Oct. 23, Gorbachev shifted his position from refusing to accept any sublimits to proposing limiting land-based intercontinental missile warheads to 3,000 to 3,300 and submarine-launched ballistic missiles warheads to 1,800 to 2,000. However, Gorbachev did not propose any overall ballistic missile total such as Akhromeyev has put forward. U.S. officials said that plan was unacceptable because of the relatively low limit on U.S. submarine-based missiles.

In other developments, State Department spokesman Charles E. Redman said the administration has received "no indications" that Gorbachev is considering extending his visit to the United States if a breakthrough is near on nuclear and space arms. A statement that such an extension was possible was made in Moscow Thursday by Georgi Arbatov, director of the Soviet Institute of the USA and Canada.

Meanwhile, Soviet Deputy Foreign Minister Alexander Bessmertnykh arrived in here yesterday for detailed preparations for the Gorbachev visit. U.S. officials said he will head a Soviet team that is to begin a new round of summit preparatory meetings next week with White House national security adviser Lt. Gen. Colin L. Powell, communications director Thomas Griscom and Assistant Secretary of State Rozanne L. Ridgway.