U.S. officials say the Soviet Union has backed away from Soviet leader Yuri V. Andropov's highly publicized pledge to "liquidate" some new SS20 intermediate-range nuclear missiles if an arms control pact can be reached with the United States.

U.S. sources said that in the intermediate-range arms talks that resumed in Geneva Sept. 6 Soviet negotiators said that pledge covered destruction only of SS20 launchers, not the missiles themselves.

"It was what we feared when it was originally announced," one U.S. official said.

Since an SS20 launcher can be reloaded after firing its initial missile, keeping spare SS20s available could leave the Soviets with a significant force advantage even after reductions, U.S. specialists say.

In a related development, sources said the Soviets have stopped removing their older SS4 missiles from service. The Soviets had been dismantling the more than 20-year-old, liquid-fueled, single-warhead SS4s as the newer, solid-fueled, three-warhead SS20s were deployed.

One State Department source suggested that the Soviets may be holding back the SS4s to be used either as bargaining chips or, perhaps later next month, as a counter to the expected NATO announcement of a reduction in the number of American battlefield nuclear weapons maintained in western Europe.

Yesterday the Senate Foreign Relations Committee was given what one source described as "a bleak report" on arms control negotiations in a closed-door briefing by Jonathan T. Howe, director of the State Department's bureau of politico-military affairs.

The committee had expected to be told the administration's position on "build-down," an idea backed by a number of influential moderates in Congress under which the United States and Soviet Union would dismantle old warheads as new ones are deployed.

Howe told the members that the administration has yet to formulate a build-down plan for introduction in the strategic arms reduction talks (START) on long-range missiles also going on in Geneva. One senator said later that he expected that to be completed in "about a week."

"Arms control is not so fashionable these days," one frustrated senator said after Howe's presentation.

The original Andropov offer was that the Soviet Union "would liquidate all the missiles to be reduced" as part of an agreement that also had to include "renunciation by the United States of the deployment" of new Pershing II and ground-launched cruise missiles in western Europe.

Last August, American officials called the Andropov offer to destroy SS20 missiles "a positive sign" even while pointing out that it did not deal with what the United States regards as the basic issue of permitting as many U.S. intermediate-range missiles in Europe as the Soviet Union has in both Europe and the Far East.

Administration officials had not expected much progress in the current round of Geneva negotiations as both sides girded for the deployment in mid-December of the first nine Pershing IIs in West Germany and the first 16 cruise missiles in Britain.

After the Soviet Union shot down a Korean airliner with 269 persons on board two weeks ago, some administration officials proposed delaying resumption of the Geneva arms control negotiations on both intermediate-range and strategic nuclear weapons.

Instead, President Reagan chose to resume discussions but make no basic change in the American position, which now calls for eventual destruction of all intermediate-range missiles, but in the interim permits the United States to have a number of such weapons equal to the total maintained by the Soviets.

At the White House yesterday, spokesman Larry Speakes said the administration would "continue an active pursuit at Geneva of arms reductions" and added that the U.S. delegation has "an amount of flexibility to listen to any serious Soviet proposals."

The administration has been studying one inter-agency proposal that would set up geographical sub-limits for intermediate-range missiles. It would have the Soviets freeze their missiles in the Far East at the current level of 108, and allow the number of U.S. missiles in Europe to equal the Soviet total west of the Ural Mountains.

President Reagan recently approved discussing that modification with the NATO allies, but has not yet included it in the instructions given American negotiator Paul H. Nitze for presentation to the Soviets. The sub-limit idea was part of the position developed by the Carter administration for the first talks with the Soviet Union, which took place in October, 1981.