The Soviets are withdrawing from Afghanistan anti-aircraft and Frog battlefield missile units that are not needed in the kind of war they are waging there, Pentagon sources said yesterday.

This intelligence substantiates Moscow's assertion that it is indeed withdrawing some troops from Afghanistan but raises a question about any significant loss of striking power.

Also, sources said, the withdrawn forces are expected to be kept close to the Afghan border in the unlikely event of a change in the character of the fighting, which started last December.

Any Soviet planner's fears about air attacks from the Afghan air force, Pakistan, Iran or the United States have not been realized. It has been a permissive environment in that sense, just as Vietnam enemy air operations were for U.S. troops.

With no threat from the skies, the Soviets are removing surface-to-air missile units, anti-aircraft gun batteries and some fighter planes, according to U.S. government officials.

Also, apparently because they see the future threat coming from small bands of guerrillas rather than standup battles with Afghan army units, the Soviets are recalling the Frog missile units.

The Frog is used like artillery. It carries a big warhead between 20 and 30 miles and is effective against big targets like airfields or troop concentrations but not against roving guerrilla bands.

The Soviet news agency Tass said Sunday that Army units were being pulled out Afghanistan that were "not necessary at present," touching off an international guessing game on how significant the promised withdrawal would be.

President Carter at first called it an "obvious propaganda effort" at a press conference in Venice on Monday. He said only about 10 percent of the 85,000 Soviet occupation troops would be withdrawn.

But on Tuesday in Belgrade the president toned down his remarks and said he was willing to explore a "transitional arrangement" in Afghanistan linked to the pullout of "all Soviet troops." The transitional arrangement would be a method of protecting pro-Soviet Afghans from bloody retribution if Soviet troops did withdraw.

While the president tried to proceed down that diplomatic track, Pentagon officials pored over the intelligence reports on the withdrawal, expected to involve between 5,000 and 10,000 troops, for clues to future Soviet military strategy in Afghanistan.

One view, sources said, is that the Soviets have given up on making a quick kill and are pulling out some heavy stuff as they resign themselves to protracted conflict unless a diplomatic exit is found.

Another view is that the withdrawal is nothing more than an attempt to put a more positive face on the Afghan occupation between now and the opening of the Moscow Olympic Games on July 20. After the Olympics, in this Pentagon view, the Soviets will step up operations against insurgents and send in whatever it takes to suppress them.

Zbigniew Brzezinski, Carter's national security adviser who was with the president's in Belgrade, took the cautious view about the significance of the Soviet withdrawal, telling reporters: "A reduction of several thousand troops, particularly those in technical units not suitable for counter-in-surgency warfare, may not be all that important.'

A United Press International dishpatch from Kabul, Afghanistan, yesterday said the Soviet pulled out more 100 tanks because they found them useless in combating the Moslem guerrillas.

As tanks leave, UPI said, more Soviet helicopter gunships -- which have proved devastating against guerrilla bands -- are flying in.

U.S. officials who take a pessimstic view of Soviet intentions stress that Moscow has told its unit commanders to expect to be in Afghanistan for two years and is amassing a support base for a long stay.