U.S. officials are determined to push ahead for NATO approval of deployment of new U.S. medium-range nuclear missiles in Western Europe despite Soviet leader Leonid Brezhnev's warning Saturday against doing so.

Officials said yesterday that the number of such missiles targeted on the Soviet Union from North Atlantic Treaty Organization bases will depend on whether Moscow agrees to limit its missile systems aimed at Western Europe.

The U.S. plan, approved last week by a high-level NATO group, is to negotiate limitations on these missile systems once deployment is under way.

Assuming that the NATO foreign ministers and their governments approve the plan, the United States will begin a slow, phased production and deployment of 1,000-mile-range Pershing II or 1,500-mile, ground-launched cruise missiles beginning in late 1983, officials said.

At the same time as production starts, the United States, representing the NATO allies, will attempt to get the Soviet Union to enter into talks aimed at reducint or freezing the number of Soviet 2,500-mile-range SS20 missile. According to U.S. intelligence sources, about 120 SS20s are deployed in Western Russia.

U.S. officials, pressing for NATO support of the medium-range-missile program, have argued that without a plan to build and deploy the new missiles, Moscow has no incentive to limit or reduce its SS20 deployments.

The longest-range U.S. nuclear system now in Europe is the 400-mile Pershing Ia system.

In his speech Saturday, Brezhnev warned that introduction of the new missiles "would change essentially the strategic situation on the continent."

U.S. officials, however, maintain that the Soviets have a monopoly on intermediate-range missile systems, having 500 SS4s with a 1,000-mile range and 90 SS5s with a 2,000-mile range.

The newer SS20s, officials said, have been replacing these earlier missiles, some nearly 20 years old.

In his Saturday speech, Brezhnev said the Soviets had not increased the number of medium-range missile launchers aimed at Western Europe and "the yield of the nuclear charges of these missiles has even been somewhat decreased."

Pentagon officials yesterday said Brezhnev's facts were correct but their meaning was deceptive.

"The SS20 has three warheads; the missiles it is replacing have only one," a Pentagon official said yesterday. "The Soviet warhead total aimed at NATO is increasing while the launchers stay the same."

He also noted that while the yields on the SS4 and SS5 were two megatons and six megatons, respectively (the Hiroshima bomb of 1945 was 12.5 kilotons, or almost 500 times smaller than the SS5), the SS20 warheads are much more accurate and 500 kilotrons each -- big enough to destroy any target in Europe.

Under the NATO plan approved last week, a target of 572 medium-range missiles was set for presentation to the NATO ministerial meeting in December. There was no final determination of how many would be Pershings and how many ground-launched cruise missiles, a Pentagon official said yesterday.

He added that the target level was far from final.

He said that the missiles were tentatively allocated among four NATO countries -- Britain, West Germany, the Netherlands and Belgium -- and that the parliaments of each would have to approve the deployments.

"If any one of them dropped out," the official said, "the total would be revised."

The final deployment also could be changed if the Soviets agreed to limitations in bilateral negotiations on a third strategic arms limitation treaty between the United States and the Soviet Union.

"It's too premature to talk about numbers now," a Pentagon official said yesterday. "We first must get some agreement on principles."