The Soviet Union has threatened to halt negotiations to reduce long-range nuclear weapons if the United States goes ahead with the currently planned deployment of new medium-range nuclear missiles in Europe, according to informed sources.

Soviet negotiators told their American counterparts during the second round of the Strategic Arms Reduction Talks (START) in Geneva last November, the sources said, that if a single new Pershing II or ground-launched cruise missile is deployed in western Europe, negotiations on reducing intercontinental nuclear weapons would come to a halt.

The Soviets thus have made the START negotiations "hostage," according to one source, to the parallel intermediate-range nuclear force (INF) negotiations that also resume soon in Geneva.

This is a much tougher Soviet line than previously disclosed. It is also significant because some U.S. officials believed the Soviets had made promising modifications in their original START position. Gen. Edward L. Rowny, chairman of the American START delegation, had said he was "guardedly optimistic" after the first two rounds of talks about achieving a worthwhile agreement.

For example, sources said, during the second round of talks, the Soviets hinted they would consider the U.S. proposal to reduce specific numbers of nuclear warheads rather than just missile launchers. The public Soviet offer was to reduce the number of missile launchers by 25 percent, without mentioning warheads. Soviet negotiators also showed interest in what are described as "confidence-building measures," such as exchanging information on defense budgets.

More recently, sources said, the Soviets appeared to be showing increasing interest in U.S. proposals for verifying compliance with any arms agreement, which they had refused to deal with earlier in Geneva. Soviet leader Yuri V. Andropov also made a public reference to adopting some new forms of verification, and international verification procedures were mentioned in a Warsaw Pact statement.

During the second round of the START talks, the United States also modified its position, sources said, by allowing heavy bombers and air-launched cruise missiles to be included in the negotiations along with missiles.

But the Soviets argued in Geneva, according to the sources, that the medium-range nuclear missiles the United States plans to deploy in western Europe should also be considered strategic weapons because they could hit Soviet cities.

The threat to quit START, the sources said, showed how far the Soviets are willing to go in building diplomatic and political pressure to prevent deployment of the new missiles, particularly the Pershing II, which could strike targets inside the Soviet Union within eight minutes of launch from bases in West Germany.

The first new U.S. missiles are scheduled to be deployed in December. If Washington and Moscow are unable to reach an agreement to the contrary, 108 Pershing IIs and 464 cruise missiles are to be based in western Europe by 1986.

In the INF negotiations to reduce the number of these medium-range missiles in Europe, U.S. negotiator Paul H. Nitze proposed during informal and officially unauthorized discussions last July with Soviet negotiator Yuli Kvitsinsky that each side limit itself to 75 missile launchers in Europe, according to an informed source.

This would allow the United States to deploy 300 cruise missiles--four per launcher, each carrying a single nuclear warhead. The Soviets would have been allowed 75 of their new SS20 medium-range missiles, each carrying three nuclear warheads. The United States would give up the Pershing missile and the Soviets would have fewer warheads.

But this approach was vetoed in both capitals.

Contrary to one published report, however, officials close to Nitze said he did not recommend in recent inter-agency deliberations in Washington that the United States forgo the entire planned deployment of the Pershing and cruise missiles if the Soviets reduced their force of new SS20 missiles to about 50 in Europe and 100 in Asia.