The Soviet Union intends to turn to mobile long-range missiles as a way of keeping its forces safe from increasingly accurate U.S. weapons under development, a senior American official said yesterday.

The official, who is familiar with the U.S.-Soviet strategic arms reduction talks (START) in Geneva, said the Soviets "have told us privately" that they know their huge force of land-based missiles in fixed silos will become "increasingly vulnerable" to the new U.S. land-based MX and submarine-based Trident II missiles.

He quoted a senior Soviet negotiator as having said "they are going to have a solution for this. They are going to go mobile" with at least a portion of the Soviet force so they can move their missiles around to make them harder to find and hit.

The U.S. official said that on balance he would view such a Soviet move as a "positive sign" rather than an escalation of the arms race because mobile missiles tend to carry fewer warheads and be smaller, less powerful and less accurate than missiles in fixed underground silos. They are also less tempting targets.

The United States is also at work on such missiles; many arms control experts say a shift to mobile weapons by both sides would make for a more stable and less threatening nuclear balance than now.

The U.S. official quoted the Soviet as saying that deployment of mobile missiles "is easy to do in our country," meaning that the Soviets have a vast land mass and no opposition from a Congress or a public fearful of nuclear missiles on the roads. The Soviet said that "they don't have the difficulties we do and it is their plan" to develop such forces, the American reported.

The U.S. official, speaking under the ground rule that he not be identified, said he "had no doubt" the Soviets would move toward a mobile missile force.

He said that a new solid-fueled missile known as the PL5, which is now being flight-tested and so far carries only one warhead rather than the multiple warheads used on some larger land-based missiles, is the likely new mobile weapon.

It would not be surprising if the Soviets turned to a mobile intercontinental missile. The increasing accuracy of U.S. missiles has been known for years and the Soviets, in recent years, have built hundreds of mobile shorter-range SS20 missiles aimed at western Europe and Asia.

But conversion of part of the land-based long-range missile force, now numbering 1,400, to mobile weapons represents a huge, costly and technically risky decision for Moscow, which has had problems developing solid-fueled missiles in the past.

The United States now has 1,050 silo-based missiles. They too are becoming vulnerable to attack or are already so.

The presidentially appointed Scowcroft Commission studying U.S. strategic arms programs and arms control efforts thus recently recommended that this country ultimately move away from large, stationary, multiple-warhead missiles such as the MX and toward smaller, single-warhead missiles that probably would be mobile.

The smaller weapons, it was reasoned, would be less vulnerable and less threatening at the same time. President Reagan accepted this recommendation and the program is moving ahead. The commission expressed the hope that the Soviets would also move in this direction.

But many questions remain.

For example, if the Soviets are determined to build mobile missiles to reduce the threat from the MX and Trident II, the official was asked yesterday, would the United States be able to pressure Moscow to reach agreement at START by continuing to build MX and Trident?

At one point, the official said: "I don't know how to answer that question." But under further questioning he said that the new U.S. weapons were needed to push the Soviets toward the smaller and less-threatening mobile missiles.

Ultimately both sides would have such weapons and fewer missiles of all types than they have today. The official said indications that the Soviets were moving toward mobile missiles were proof that Reagan administration plans were working. And Moscow undoubtedly would always keep some bigger silo-based missiles, so a countering force would also be needed by the United States, he said.

The official said, however, that whether mobile weapons ever represent less of a threat than today's weapons depends upon the total number of warheads allowed on each side. Thus far, the Soviets at Geneva have not indicated how many warheads they would be willing to agree on. The United States has proposed that each side limit itself to 5,000 warheads, roughly one-third below current levels.

The official also acknowledged that even a START agreement on U.S. terms would not necessarily remove the ability of either side to launch a successful first strike against the other.

Five thousand warheads might still be enough to allow either side in a first strike to wipe out a large part of the other side's forces.

The official said the existing vulnerability of U.S. land-based missiles would therefore not be solved entirely by an arms agreement. It would also require this country either to build mobile missiles or to find better ways to protect fixed-based missiles.

The official "reserved" opinion on whether the Soviets have violated past arms agreements, but said the Soviets were "short-sighted" if they were violating pacts because they would achieve only "small possible gains and risk big possible losses."

The White House recently asked for a special meeting with the Soviets to explore three alleged breaches of previous agreements.

The official revealed that the Soviets have begun testing a new version of their existing SS19 missile with 10 warheads, the same number as the MX will carry. The Soviets have 330 SS19s, most of which now carry six warheads, and 308 SS18 missiles with 10 warheads each. The advanced U.S. Minuteman III missile has three warheads.

The official said he remains hopeful that a START agreement can be reached because "in the long run, they the Soviets want one."

The official confirmed many recent shifts in the positions of the two countries at the talks.

Aside from the 5,000-missile-warhead limit, the United States is proposing a limit of 400 bombers each with no more than 20 air-launched cruise missiles aboard each bomber.

In an important recent shift, the United States would drop demands that Moscow reduce its big edge in throw-weight, or the lifting power of its rockets, to levels below the current U.S. level. Now, the administration would accept a reduction to a level higher than the U.S. level. This still means a substantial Soviet cut, but U.S. officials have not said how much they would settle for.

The Soviets, rather than dealing with warheads, have proposed limiting each side to 1,080 multiple-warhead missiles, 680 of these based on land, and 120 bombers.