Soviet mountaineer and counter-guerrilla units are being sent into Afghanistan for the first time, U.S. defense officials said yesterday.

These officials, on the basis of the latest intelligence reports, said the changing character of the Soviet occupation force does not mean any significant increase in the total number of troops in Afghanistan because the Soviets are rotating their forces.

Shortly before Christmas, the Soviets ordered into Afghanistan units stationed near the Soviet-Afghan border, fleshing out the ranks with reservists. Many of those reservists, according to the Pentagon, are not being sent home and replaced by regular Soviet troops from units farther inland.

The rotation, officials said, is being wrongly interpreted in some published reports as evidence of a Soviet escalation by committing a larger force to Afghanistan.

Intelligence officals also said there is nothing alarming or unusual about Soviet troop exercises in Eastern Europe, declaring that the massing of Russian and East German troops in Hungary 60 miles from the Yugoslav border does not appear to be related to the illness of Yugoslav President Josip Broz Tito.

The same calm interpretation was put on the Soviet Union's deployment along its border with Iran, with U.S. officials asseting yesterday that there is no evidence the Russians are preparing a thrust into Iran, as some have speculated.

What does seem to be happening, defense officials said, is a stepped-up Soviet effort to "pacify" Afghanistan, to use a term from the Vietnam war.

The detection of Soviet mountaineer and counterguerrilla forces going into Afghanistan is being interpreted by some U.S. military specialists as evidence that the Russians high command has concluded that it cannot depend on the Afghan army to combat the rebels.

What was termed "sporadic" fighting between Soviet and Afghan army units is continuing, officials said yesterday. The Afghan army has decreased from 100,000 to 50,000 troops, according to the Pentagon, with many of those defectors joining insurgent units. Others just went home, officials said.

U.S. intelligence sources agree that much of the Afghan army's weaponry and equipment has been lost in the defections. But how much of it ended up in the hands of the disorganized rebel forces is unclear, officials said.

The standard tactic the Soviets are using in Afghanistan, officials said, is to send Afghan army units into towns ahead of Russian troops. But often this has not provided the desired security, forcing the Russians to take a heavy hand in quelling resistance in some towns.

U.S. intelligence agencies recently received evidence that the Soviets have sent into Afghanistan trucks used to decontaminate troops exposed to gas, and Frog surface-to-surface missiles.

This combination raised the possibility, but only that, of the Soviets resorting to disabling or poison gas to combat rebel tribesmen in the mountains. The Frog can carry chemicals as well as conventional explosives in its nose.

However, the decontamination trucks and Frogs could represent nothing more than a standard deployment of Soviet forces, Pentagon officals acknowledged.

Military officials sitting through the intelligence reports on Afghanistan and pondering the Soviet Union's military options said they range from "hanging in there through the winter and grinding down the guerrillas" to taking immediate, aggressive action against resisting units by sending the mountaineer and counterguerrillas into the hills.

One U.S. intelligence estimate is that there are about 75,000 Soviet troops in Afghanistan and 25,000 more just over the border.