The United States and the Soviet Union have hardened their negotiating positions on medium-range nuclear missiles, dimming the prospect of a breakthrough in the Geneva arms talks and promising new strains in December when the first new U.S. Pershing II and ground-launched cruise missiles are to be deployed in western Europe.

With talks scheduled to resume in Geneva on May 17, both sides appear to be preparing for how they will react to deployment rather than trying to reach agreement to head it off, according to sources inside and outside the administration.

If the initial nine Pershing II missiles are placed in West Germany and 32 cruise missiles in Great Britain and Italy as scheduled, sources said, the Soviet Union is expected to break off the Geneva talks on intermediate-range nuclear missiles.

U.S. officials believe, however, that the Soviets will eventually return to bargaining because, as one top administration official put it, they do not want to see all 572 new U.S. missiles deployed and cannot afford to alienate public opinion, particularly in western Europe, by abandoning arms-control efforts.

Diplomatic sources said the Soviets might not return until they have taken some new step in the arms race, however. That is because, as one source said, U.S. deployments will create a new nuclear threat to which civilian leaders in Moscow, under pressure from their military, must respond.

Publicly, Soviet officials have threatened to deploy a "comparable" nuclear missile if U.S. systems are installed in Europe. The Pershing IIs could strike targets inside the Soviet Union within about eight minutes of being launched from West German bases.

Comparability, according to Soviet sources, would involve nuclear missiles that could hit targets on U.S. soil within minutes of launch. Although some Soviet commentators have hinted that this could mean putting missiles in Cuba, Nicaragua or El Salvador, informed sources here rule out such a move, saying Soviet leaders realize that such a step would be too provocative.

Instead, U.S. officials said they are looking more at the prospect that Soviet SS20 missiles, with a range of several thousand miles, may be stationed in the eastern Soviet Union from which they could hit Alaska and perhaps the state of Washington or that the Soviets could deploy additional missile-firing submarines or ships off the U.S. coasts.

In private talks in Geneva, the Soviets have also threatened to pull out of the strategic nuclear arms negotiations, but the U.S. view is that they will not, U.S. officials said.

Given the current situation, little work is being done in the administration, sources said, to expand President Reagan's March pronouncement that he is prepared to have the Soviets set some interim missile warhead limit applicable to both sides.

The original U.S. plan, supported by the British, was to limit U.S. and Soviet medium-range missiles to no more than 300 warheads and 100 launchers on either side. But the numbers were dropped at the suggestion of West German Chancellor Helmut Kohl, sources said, to force the Soviets to pick a number between zero and 572, the number of warheads planned for the full NATO missile deployment program.

To date, however, the Soviets are holding fast publicly and privately to the position that no U.S. missiles should be deployed, sources said.

A key to any interim agreement breakthrough, administration and diplomatic sources say, is dropping deployment of the Pershing II.

U.S. negotiator Paul Nitze proposed dropping the Pershings in his unusual attempt last July to lay out the framework of an agreement with his Soviet counterpart.

When Moscow indicated that it was not prepared to accept a low number of cruise missiles only, Washington turned against the Nitze notion of dropping the Pershings.

Recently, amid indications that the Soviets might reconsider, Washington made some discreet inquiries, administration sources said last week. "They were not interested," a U.S. official said yesterday.

U.S. officials went public last month, saying the deployment had to include a mix of Pershing and cruise missiles.

Last Tuesday, the new director of the Arms Control and Disarmament Agency, Kenneth L. Adelman, caused some confusion by telling western European reporters here that the United States would consider "a serious Soviet proposal," including one to eliminate the Pershing II.

"The kind of mix and the kind of systems," Adelman said, "should be worked out by our negotiators in Geneva." Faced with such an offer, he said, he would not "want to indicate one way or the other what . . . my recommendation to the president would be on eliminating the one system . . . Pershing IIs or not the Pershing IIs."

After Adelman restated that approach three times in the interview, one journalist reminded him that other Reagan officials recently "had ruled out giving up the Pershing II."

"I would agree with that statement," Adelman said, although he still maintained that, if given a serious Soviet proposal eliminating the Pershing but allowing cruise missile warheads in numbers equal to Soviet SS20s, "we would negotiate, we would talk about it."

When European press reports appeared Wednesday saying Adelman would be willing to give up Pershing II deployment as part of a negotiated agreement, the State Department issued a statement saying, "This is an incorrect translation of Ambassador Adelman's remarks."

The one-hour, on-the-record European news conference, Adelman's first as ACDA director, took place after his aides recently told U.S. journalists that he would not be talking on "substantive matters" for at least a month.

Adelman began the session by saying he wanted to "stay in touch with the European press . . . your concerns are our concerns," and he added that he is very concerned with "European public opinion."

In a reference to the recent bitter fight over his Senate confirmation, Adelman said, "I decided after three months of quite a bit of press involvement that I would really hold off on doing many interviews or any interviews for the first few months with the American press."

But he added quickly, "I did think it was so important to do this kind of interview with the European press."