The Soviet Union has launched its biggest military operation in Afghanistan in two years, with 10,000 Soviet and Afghan troops fighting their way along a mountain road to break U.S.-armed guerrillas' siege of the long-isolated garrison town of Khost, according to Pentagon officials.

The unusual winter offensive at a time when Soviet leaders are talking increasingly about withdrawing their troops from Afghanistan has puzzled U.S. military analysts. They say there is no way the Soviets can keep open the mountain road, which has been closed for seven years, even if the combined Soviet-Afghan force reaches Khost.

"It's an object of some mystery to us why they are doing it," remarked one Defense Department official. "This is definitely a tough nut to crack and militarily it doesn't make much sense.

"Maybe while withdrawing they want to show they are not going to be run out of the country, that this is a good way to show your resolve," the official added.

Other sources said the Soviets, who have had great difficulty in resupplying the Khost garrison by air this year because of the Afghan rebels' vastly improved antiaircraft weapons, may have feared that losing the city outright to the rebels would have been a serious blow to the shaky Soviet-backed Kabul government.

Pentagon officials said an Afghan army division is based in Khost. Afghan President Najibullah, announcing on Nov. 29 his government's intention to break the Khost siege, said 40,000 people were trapped there.

Defense and State Department officials said the Soviet-Afghan offensive, apparently the largest since those conducted in the Panjsher Valley in 1985, reflects a change in Soviet tactics toward new reliance on long-range heavy artillery, instead of warplanes and helicopter gunships, to protect their troops from the increasingly well-armed Afghan resistance.

The offensive, now three weeks old, involves a Soviet attempt to push through to Khost from the Paktia provincial capital of Gardez in southeast Afghanistan. The advancing armor and troop columns have been held up primarily by a twisting, heavily mined 15-mile-long mountain pass that has been under attack from 8,000 to 10,000 Afghan resistance fighters camped in the hills above the road.

The Pentagon official said that although the Soviets portray the Khost campaign as an Afghan army operation, half the troops were Soviets. Of the estimated 120,000 Soviet troops now in Afghanistan, only about 30,000 are available for combat operations, according to Pentagon estimates.

A State Department official said Monday that the Soviets have backed up the Khost campaign with "very heavy artillery support," which is being used in place of intensive close-air support.

U.S. analysts say the change was forced on the Soviets because effective rebel use of U.S.-built Stinger and British-made Blowpipe antiaircraft missiles has gone a long way toward neutralizing Soviet-Afghan air power. The shoulder-launched, heat-seeking missiles have raised the risks sharply for Soviet and Afghan government warplanes and helicopter gunships flying at low altitude.

A State Department report issued this week on the eighth year of the Soviet occupation estimated that total Soviet-Afghan losses from rebel antiaircraft missiles and guns this year were "in the range of 150-200 aircraft" and that "for some periods" the rate reached one or more a day.

"These losses force the Soviets to reevaluate their tactics," the report said, resulting also in "a marked decrease in Soviet air activity with a resulting drop in air losses."

The supply of Stingers to the seven Afghan resistance groups being armed by the United States surpassed 1,000 units this year, according to two knowledgeable sources.

Pentagon and State Department officials said the Soviet-Afghan relief force had been making slow but steady progress and should reach Khost, about 15 miles from the Pakistan border, in the near future.

But one Pentagon official said the Soviets would be "crazy to stay there" and would need "10,000 men to hold the road open behind them."

The fighting has resulted in another wave of Afghan war refugees streaming into neighboring Pakistan, with Pakistani diplomatic sources here reporting between 10,000 and 15,000 new arrivals in the past few weeks.

Despite Soviet suggestions of an imminent withdrawal, the State Department report and Pentagon officials said no indication on the ground has been detected that such an action is being prepared. The report said Soviet combat forces may have actually increased this past year, with the arrival of a few new artillery units and reinforcements for other units already there.

The report also said the Soviets continue to improve their military infrastructure. "Airfield runways are being improved, revetments made for aircraft, and fuel and ammunition storage sites expanded to include the construction of mountainside and underground bunkers," it said.

The Soviets also continue to build up security zones around Kabul and other major cities. "These measures indicate Soviet plans to depart the country, if any, have not yet been reflected in actions on the ground," the report said.