A new proposal for limiting medium-range nuclear missiles in Europe that President Reagan will unveil at the United Nations on Monday contains an American offer to discuss a deal with the Soviet Union in which the United States might not deploy all the missiles allowed under such an agreement.

The forthcoming proposal, which administration officials acknowledge to be complicated and lacking many details, is meant to give greater flexibility to U.S. negotiators at the Intermediate Nuclear Forces (INF) talks at Geneva and to "address a number of Soviet concerns," according to a statement the president issued Thursday.

Reagan described his forthcoming proposal as a "significant further development" beyond the original U.S. position and its subsequent modifications.

Although details of the new proposal have not been made public here, they have been given to Soviet delegates at Geneva. Their initial reaction, as reported Thursday by the Soviet news agency Tass, was that it "strongly smells of the old, odious pseudo-zero option" proposal offered by the United States after the talks began in November 1981.

Under that proposal, the United States would forgo deployment of 572 new Pershing II and ground-launched cruise missiles, scheduled to begin in December, if the Soviets would scrap about 600 existing missiles, including 351 modern SS20s deployed in European and Asian portions of the Soviet Union.

The United States has since modified its proposal and offered an interim agreement with the Soviets in which both sides would agree to equal, global limits, which would include Soviet missiles in Asia and Europe, at some level below the 572 missile warheads planned by the United States.

Although each U.S. missile is to have one warhead, the Soviet SS20s each carry three. The SS20s also are mobile and could be moved from Europe to Asia and back again.

Officials said the new U.S. proposal still calls for equal, global numbers of missile warheads, allows no SS20 movement between Europe and Asia and does not include 162 British and French nuclear missiles the Soviets want counted in the NATO alliance total.

But three things are new. As the officials explained it, if the two sides can agree on an equal number of warheads, the United States is prepared to discuss deploying fewer missiles in Europe than it would be allowed.

For example, one official said, if the two sides agreed to deploy 200 medium-range missile warheads, and the Soviets chose to deploy 100 in Europe and 100 in Asia, the United States would consider not deploying all of its 200 missiles in western Europe.

The 200 figure is just an example, the official said. There will be no numbers in the president's speech, and the precise level would be negotiated. The administration is also not prepared to say what, if anything, it would do with the extra missiles, although one official said they could be kept in the United States and deployed in an emergency.

Of the 351 Soviet SS20 missiles, 243 are in Europe and 108 in Asia. It has been widely reported in the media that if the Soviets agree to missile warhead parity in Europe, the United States will agree under the new proposal to let the Soviets keep the 108 missiles in Asia. U.S. officials say that is not accurate.

"We have made no such commitment," one official said. He also pointed out that the arithmetic of the situation would almost certainly force the Soviets to reduce forces in Europe and Asia.

For example, if the Soviets kept 108 of the triple-warhead SS20s in Asia, a total of 324 warheads, they would have much less to deploy in Europe against NATO forces under an overall ceiling of 572 or less.

Officials confirmed yesterday that the other new aspects of the proposal, as previously reported, include an American willingness to consider cutbacks of both the Pershing II and cruise missiles, rather than just the slower-flying cruise missile, if any agreement is reached on overall reductions.

In addition, Washington will agree to include in the negotiations aircraft of both sides based in Europe and capable of carrying nuclear bombs, although there is disagreement about the number of such planes in each arsenal.

The fact that Reagan is publicly introducing new arms control efforts despite the Soviet downing of a South Korean airliner Sept. 1 is evidence, officials have said, that the president remains committed to dealing with the overriding issues of nuclear arms control.

But other officials said several other elements of U.S.-Soviet relations will probably hold the key to whether progress is made.

One key factor is that previously planned meetings between Soviet Foreign Minister Andrei A. Gromyko and Secretary of State George P. Shultz have been broken off and no one is sure when or if they will be resumed.

In effect, officials said, it requires high-level contact above the level of the negotiators at Geneva to cement any chance for a breakthrough and to set the stage for a possible summit meeting between Reagan and Soviet leader Yuri V. Andropov that would announce even limited arms control agreements. Before the airliner incident, there were some preliminary estimates, officials said, that a summit might take place next spring or summer.

Another factor is that Reagan's condemnation of the Soviets and their system continues at a high level. While Moscow may be more interested in Reagan's actions than his words, some specialists believe the Soviets take the attacks on their system seriously. Thus, specialists said, dealing with the Soviets may depend on when, if ever, the furor over the airliner dies down.

Officials said that Reagan is under different kinds of pressures in the INF talks than in the companion talks in Geneva dealing with long-range missiles and bombers that are based in each superpower's homeland. These are called the Strategic Arms Reduction Talks, or START.

U.S. allies in Europe have a large stake in the INF negotiations and want continued momentum there despite the downing of the Korean 747. Thus, allied pressure on the Reagan administration continues.

But because the sense of outrage over the airliner incident is so strong and prolonged in the United States, Reagan is viewed as under much less pressure at home in the START talks, which deal with American and Soviet weapons aimed at each other.

Nevertheless, officials yesterday said Reagan remains committed to pursue those talks as well. As evidence, the White House released a letter yesterday from the president's national security affairs adviser, William P. Clark, to a group of congressional moderates, assuring them that their suggestions for making the U.S. START proposal more negotiable are being worked into the American stance and that this integration should be completed next week, before the START talks resume on Oct. 6.