The Soviet Union has delayed retiring 20-year-old medium-range nuclear missiles, which are aimed at Western Europe, so they can be included in any future arms limitation agreement, according to White House sources.

That conclusion, they said, emerges from an analysis of how the Soviets are introducing their newest medium-range missile, the SS20, and dismantling aged SS4s and SS5s.

It is against this background, these sources said, that Soviet President Leonid Brezhnev's offer to reduce "medium-range nuclear means" deployed against Western Europe was taken by the White House more as an effort to halt approval of a NATO medium-range missile than a real arms control offer.

Almost all of the estimated 140 older missiles that have been dismantled in recent years were at sites in central and eastern Russia and targeted on China, sources said.

At the same time, sources said, SS2s, which carry three warheads, are being deployed almost equally at sites that can hit either European or Chinese targets.

The result, they noted, is that there now has been an sharp increase in Soviet nuclear firepower aimed at NATO targets with some of it being old missiles of the type that had been retired from sites along the Sino-Soviet border.

Brezhnev's offer is not the first time one of the superpowers has offered to cut back on a nuclear weapon it already considered surplus to military needs.

The United States, in the early 1970's, proposed a cutback of 1,000 nuclear warheads in Europe if the Soviets would remove one of their tank armies.

To back up that offer, U.S. military men were ordered to keep in stockpiles almost 1,000 warheads for the Honest John missile although the rockets had been sent back to the United States for destruction.

"The Brezhnev offer last week to cut back on Soviet nuclear systems would come to pass, he said, "only in the event if no additional medium-range means are deployed [by the United States] in Western Europe."

The United States is pressing NATO allies for approval in December of a plan to produce and begin deployment in late 1983 up to 572 medium-range cruise and Pershing II missiles that could reach targets in the Soviet Union from NATO bases.

The decision to go ahead with the new systems would be accompanied by an offer to begin U.S.-Soviet negotiations that would limit the medium-range nuclear systems maintained by both superpowers.

The Soviet government in turn has begun a major effort to head off NATO approval of the new missiles. Soviet spokesmen maintain that although NATO does not now possess any European-based missiles capable of reaching Russia, there is already a nuclear balance in the European theater.

White House officials see the Brezhnev offer as an attempt to undermine the determination in some NATO countries to go ahead with the new missiles by offering a meaningless Soviet promise to cut back on nuclear systems.

That feeling was behind President Carter's statement last week that the Soviet leader's proposal was "not quite as constructive . . . as at first blush it seems to be . . . ."

Although the NATO allies have a large number of short-range nuclear launchers, the longest range missile system now in Europe is the Pershing I which can travel 400 miles. That is not far enough to reach targets in the Soviet Union.

On its side, the Soviet Union now has, according to intelligence sources, over 500 medium-range missles in western Russia that can hit Western Europe.

Of some 100 SS20s now deployed, two-thirds are aimed at NATO targets from sites in the Soviet Union, according to the Senate commitee report. The rest are targeted on China.

According to sources, each SS20 launch vehicle can fire and then reload with another three-warhead missle. Thus it provides many more warheads than the older SS4 and SS5 single warhead missiles it replaces.

U.S. and many NATO officials argue that introduction of the SS20 and the modern Backfire Soviet bomber have given a significant nuclear advantage to Mosgow that must be countered.

They also see a political need for the NATO countries to back the new missile systems after last year's debacle over the proposed introduction of artillery and short-range missile systems with neutron explosives. At that time, President Carter deferred the new weapons -- which also were the target of Soviet attacks -- when no decision could be reached on where they would be based in Europe.