Resistance fighters known as mujaheddin have opened a counteroffensive against Soviet and Afghan Army forces in Afghanistan's strategic Panjshir Valley aimed at recapturing positions lost earlier this year.

Attacks by well-organized guerrilla units commanded by Ahmed Shah Massoud have focused on the Afghan Army-held town of Puzhgur in the middle reaches of this hotly contested valley and have sparked renewed heavy fighting. The counteroffensive comes near the end of a year that has seen the highest level of fighting since the Soviet invasion of late 1979 in the now devastated and largely unpopulated valley.

Puzhgur's garrison of about 500 men has been cut off since late September from resupply by either land or air. Guerrilla units have continued to ambush convoys moving among four other communist enclaves farther down the valley.

Puzhgur, about 18 miles from the mouth of the nearly 60-mile-long Panjshir, marks the farthest extent of communist advance up the valley. A Soviet force stationed at the base withdrew in September, leaving its defense to the Afghan Army. But while some key outposts have already fallen to theresistance fighters, mines around government positions are hampering guerrilla efforts to press home their attacks.

In April, Soviet and Afghan Army forces launched their seventh major offensive on the resistance stronghold, ending a year-long truce. The assault involved massive aerial bombardment and extensive deployment of helicopter-borne commandos in addition to an armored advance along the valley floor.

From a bridgehead at the village of Anawa near the valley mouth, communist troops advanced to establish new strongholds along the valley floor at the towns of Rokheh, Bazarak, Bahrak and Puzhgur.

But Afghan government reports that the Panjshir -- a springboard for guerrilla operations against the country's sole north-south road link and Bagram air base -- had been completely pacified were belied by continued clashes. And in September, Soviet forces launched another major offensive -- the eighth -- against elusive guerrilla units operating along the length of the valley.

Soviet gains appear to have been limited. Advance warning of the April drive prompted an evacuation of the valley's civilian population and most guerrilla forces. But by May, rebel units had filtered back into Panjshir to begin harassing communist positions and communications along the valley's single dirt road.

Mujaheddin casualties appear to have been light, in sharp contrast to casualties suffered by attacking forces. In a recent interview, commander Massoud said his men had suffered 50 killed in the seventh offensive and 13 in the eighth. Altogether this year, about 150 mujaheddin in the valley had been killed, he said.

Massoud estimated communist casualties in the valley and surrounding areas at more than 2,500 killed and wounded in the seventh offensive and more than 3,000 in the September operations. Most of these were Soviets, he said.

It is impossible to confirm these figures independently. But judging from a wide range of local accounts, Soviet losses do appear to have been surprisingly heavy in ambushes in the Darrah side valley during the seventh assault and again during a copter-borne landing at the town of Khenj in September.

Despite continued daily bombardment by Soviet SU25 and MiG23 jets, guerrilla forces currently enjoy almost complete freedom of movement in daylight hours in areas of Panjshir upstream from Puzhgur. They also control high ground and smaller side valleys overlooking other communist strong points and stage ambushes along the road.

Rebel commanders now argue that the absence of a civilian population -- once estimated at about 100,000 -- has freed them in hitting back at Soviet forces.

"We no longer have to worry about retaliation against the villages," said one of Massoud's aides.

Villagers who left the valley before the April offensive are now living as refugees in neighboring valleys, in Kabul, and, to a lesser extent, northern Pakistan. While skeleton agriculture is being maintained by local guerrillas in some villages visited by this correspondent, fields and villages along the valley floor have been laid waste in a systematic scorched-earth campaign.

The current counteroffensive began Oct. 9 with an assault on a mountain position above Puzhgur. The post was overrun within half an hour, and most of its Afghan Army garrison surrendered. A second outpost was captured the following day amid attacks on the main base.

But the difficulties faced by the attackers were underscored when another assault on a key outpost above the town bogged down in a mine field two days later.

Mines have now become the most serious obstacle for guerrillas, who have learned to take tanks, helicopter gunships and jets more or less in stride. A growing proportion of Panjshir's casualties now are caused by mines.

Mine fields have been laid in depth -- and frequently reinforced -- around all communist strong points. In addition to buried pressure mines, defenses consist of networks of antipersonnel mines on raised wooden supports and detonated by interconnected tripwires.

Guerrilla tactics have relied on throwing in small assault squads armed with hand grenades and grappling irons. The irons are hurled forward on the end of ropes and then pulled back to catch on trip wires and detonate mines before the attackers crawl forward under supporting fire from mortars, recoilless rifles and heavy machine guns.

As witnessed during one assault, the art of mine-kashi, or "pulling the mine," is a difficult one and, for the inexperienced, almost suicidally dangerous. Two resistance fighters were killed by mine fragments to the chest when kneeling to gain greater leverage on their grappling irons rather than pulling from a prone position.

But despite high morale among the guerrillas, it remains doubtful whether the current assaults will succeed in pushing Afghan government and Soviet forces from the valley before winter.

According to Massoud, 14,500 communist troops are deployed in Panjshir: 7,000 Soviets at Anawa, Rokheh and Bazarak and an additional 7,500 Afghans at Bahrak and Puzhgur.

Massoud declined to give figures for his own forces, but independent estimates suggest that between 5,000 and 10,000 full-time guerrillas are active in the valley.

Indications are that the rebel successes will hinge largely on rock-bottom morale among Afghan Army units composed mostly of conscripts, many of whom defect at the first opportunity, often in the confusion of an attack on the position they supposedly are defending.