The Antonov spotters begin a lazy circling of the snow-capped upper reaches of the Panjshir Valley in the hours after dawn. Amid the tangle of jagged peaks the drone of the twin-engined reconnaissance aircraft signals the beginning of another day's lethal hide-and-seek between Soviet aviation and caravans of horses, mules and men bringing munitions to this embattled valley.

Of special interest to both Antonovs and the Su25 ground-attack jets that often follow them is the Chamar Pass. The 16,000-foot-high pass, wedged between some of the highest peaks of the Hindu Kush, forms the northern gateway to the Panjshir from the Nuristan region of eastern Afghanistan. It also marks the last major natural obstacle in what is almost certainly the longest and most arduous logistics lifeline of any fighting force in the world today.

Although the rebels say most of their weapons are captured from Soviet or Afghan forces, they are known to receive assistance from several countries and much of that is thought to be indirectly financed by Washington.

For the Moslem resistance of the Panjshir, probably the best-organized and hardest-hitting center of anti-Soviet resistance in Afghanistan today, that lifeline is assuming a growing importance.

From sanctuaries in northern Pakistan, convoys of from 50 to 500 pack horses and donkeys loaded with ammunition and weapons take about two weeks to trek west across Nuristan to the Panjshir to bring supplies to the Moslem fighters known as mujaheddin. Guerrilla groups moving beyond to the provinces of Badakhshan, Takhar and Kunduz along the Soviet border may spend up to a month reaching their home fronts.

For most of the journey, the terrain presents far more daunting a challenge than the Soviet Air Force. Between the Pakistani border and the Panjshir, arms convoys cross at least four major mountain passes at an average altitude of about 15,000 feet. Snow-covered for much of the year, the passes become completely blocked from mid-November through April.

Demands on both animals and men are extreme. The bleached skeletons and rotting carcasses of horses that litter steep mountain trails are stark evidence of the toll taken by sheer exhaustion -- horses often carry loads of close to 180 pounds -- and treacherous, ice-covered rocks.

Food, too, is scarce. In the sparsely populated valleys of Nuristan, one of Afghanistan's remotest regions where agriculture hardly rises above subsistence level, supplies are difficult to purchase. If and when they are available, basic commodities can fetch more than double the price normal elsewhere.

While traveling with guerrillas to the Panjshir, I walked more than 12 hours a day, often on only one meal of dried bread and tea. The drastically reduced diet was one most of the guerrillas appeared to have adapted to with relative ease.

Soviet efforts to interdict the supply trail have concentrated mainly on the areas close to the Panjshir itself. But for the most part, results have been limited by the vastness of the terrain and difficulty of pinpointing targets.

As the mujaheddin have learned, concentrations of horses can expect to attract almost immediate airstrikes. In early September, on the approaches to the Chamar Pass, Soviet jets attacked an encampment of nomads, killing more than 25. The nomads had apparently been mistaken for guerrillas moving toward the Panjshir.

Simple but generally effective countermeasures taken by guerrilla convoys reduce the risks substantially. Larger caravans are usually split up.

"We are well spread out over several days," said Ali Akbar, a resistance fighter returning to the valley with a caravan of more than 400 pack animals. "At most, a jet attack may catch 20 horses. But even that is unlikely."

Areas of high risk, in particular the approaches and saddles of passes into the valley, are now usually traversed under cover of darkness, and incidents like the attack on the nomads appear to be rare.

Soviet "butterfly" mines, sown liberally across known resistance trails, have ceased to be more than a minor irritant. Now easily recognized, the miniature antipersonnel devices, designed to blow off a hand or foot, can be cleared from well-trodden tracks within hours of being dropped.

As Soviet pressure has tightened on other major logistics routes since the beginning of the war, indications are that trails across the far-flung mountain ranges of Nuristan are becoming increasingly important to resistance in the Panjshir and Afghanistan's northern provinces generally.

In May 1982, Soviet copter-borne commandos captured the key settlement of Koran va Monjan to the northeast of the Panjshir, a surprise assault that closed off a major northern supply route to the valley and the sprawling northeastern border province of Badakhshan.

This year, a tightening of Soviet security near the Kabul River to the south of Panjshir also disrupted logistics and resulted in increased use of trails across Nuristan hitherto regarded as impractical. But guerrilla commanders appear confident that given the extreme terrain and the distances from Soviet bases, any serious interdiction of the new routes will prove far more difficult.

The flow of munitions reaching the Panjshir across Nuristan is clearly proving a vital supplement to quantities of weapons and ammunition being captured in continued heavy fighting here.

Both senior resistance commanders and the rank and file remain insistent that outside support reaching the valley is barely sufficient for effective defensive operations against overwhelming Soviet numbers and firepower.

In April and September this year, Soviet and Afghan army forces launched major offensives against the guerrilla redoubt.

The assaults -- the seventh and eighth since the Soviet invasion of Afghanistan in late 1979 -- were the heaviest to date and established communist enclaves at five points along the lower Panjshir Valley.

"The means we have at present do not enable us to coordinate our activities and go onto the offensive in the way we would like," said Mohammad es-Haq, a rebel political officer.