Soviet officials, while agreeing that President Reagan's proposals for cutback of nuclear-armed missiles warrant further discussion, criticize this latest U.S. approach as "unfair and unrealistic" and aimed at seeking advantage over Moscow.

These well-informed Soviet officials, who criticize Reagan's motives and question his sincerity, offer preliminary assessments of the substance of his proposals that discourage optimism for speedy progress in the talks that President Leonid Brezhnev said today should indeed go forward.

One of the problems, from Moscow's point of view, is that Reagan's approach departs from a common ground worked out in protracted and difficult negotiations with three previous administrations over more than 10 years.

The Soviets say the prospective talks should have as their starting point the 1979 Soviet-American Strategic Arms Limitation Treaty, SALT II.

Realizing the unlikelihood of SALT II being ratified by the Senate in the face of Reagan's opposition, Soviet officials say "the most positive aspects" of that treaty have to be preserved. These unspecified aspects--as well as an agreement, at the time SALT II was signed, on principles for subsequent strategic talks--would have to provide the basis for the forthcoming negotiations, in Moscow's view.

Apart from conceptual differences, the Soviets indicated they could not go along with the framework advanced by Reagan that would involve reductions in their principal deterrent--the land-based missile force--while the United States would continue to develop its missile-carrying submarines, bombers and the cruise missile, which could be based near Soviet territory and effectively tilt the strategic balance.

Reagan, at a press conference four days after the May 9 speech laying out his plan, declared that "nothing is excluded" from discussion under his proposal. But he stressed concern with the long-range missiles as the most destabilizing. Suggestions from Washington that Reagan would scale down research and production of the other nuclear-weapons systems does not appear to be taken seriously here.

The core of the proposal--Reagan's call for each side to cut to about 5,000 the number of missile warheads, with no more than one half of those to be land-based--is described here as a public-relations ploy. It is this that the officials described as "unfair and unrealistic."

They said it would raise troublesome verification problems that plagued SALT II, although the basic units to be counted in that agreement were launchers, large missiles that can be spotted by spy satellites.

The sources pointed out that 75 percent of Soviet warheads are located on land, with the remaining 25 percent on submarines and aircraft. In the case of the United States, the situation was "precisely the opposite," they said.

The Soviets noted American concern that the new generations of Soviet land-based missiles are so powerful and accurate that they could overwhelm the smaller Minuteman missiles.

But in Moscow's view, the Americans were trying to change the rules of the game to correct a decision made two decades ago: to opt for the smaller but accurate Minuteman apparently on the assumption that the Soviets would not be capable of improving their huge SS11 rocket.

While the Minuteman was solid-fueled and accurate, the SS11, with a huge payload, was what experts call a "dirty" weapon, using liquid fuel and lacking in accuracy.

Soviet sources now argue that they needed the huge rockets because they had no other means at the time to offset America's technological lead that gave the Minuteman its high degree of accuracy.

In the meantime, according to this view, the Soviets have improved their rocket forces by developing solid fuel and increasing the accuracy of their fourth-generation SS18 (and also two other models).

While Minuteman III carries six independently targeted warheads, SS18 is armed with 10. Because of its huge throw weight, the SS18 could with some modifications carry up to 28 warheads.

But as one official familiar with Kremlin strategic thinking put it, "Now we do not need throw weight anymore" because of improved accuracy. He suggested room for negotiations in this area.

The second point of Reagan's proposal suggested a negotiated "equal ceiling" on the throw weight of all missiles carrying nuclear warheads.

The sources here, while noting "bits of a positive approach" in the president's public plan, nevertheless, remain pessimistic about his administration. They characterize it as reflecting the darkest attitudes of American politics, with resilient hostility toward the Soviet Union and worrisome hunger for military superiority.

The sources emphasized that these were preliminary assessments of Reagan's recent statements and that his private letter to Brezhnev was being studied carefully.