Soviet and Afghan government forces trying to cut critical supply routes of the resistance movement inside Afghanistan are threatening the main guerrilla base in eastern Paktia province, according to Afghan sources in Pakistan.

Several of the sources, reached by telephone, suggested that the mujaheddin, or resistance fighters, in Paktia were in a more desperate position than during previous Soviet offensives in the province. They also described the fighters as demoralized, especially over their lack of effective air defenses, despite reports that the Reagan administration has decided to supply sophisticated antiaircraft missiles to them.

The sources, located in Peshawar and Miramshah, two towns near the Afghan border, said the Soviet and government troops, backed by artillery and air support, have inflicted heavy losses on the resistance fighters in their first offensive against the supply trails since the end of winter.

Soviet-led forces also reportedly have killed or wounded hundreds of rebels and civilians in the southern city of Kandahar, according to Afghan sources in the Pakistani town of Quetta.

In Paktia, the heaviest fighting is centered around Jawar, a major resistance base about six miles from the border with Pakistan, near Miramshah.

There have been conflicting reports about the status of Jawar, a sophisticated network of caves dug into canyons and hillsides that serves as a major staging point for essential supply caravans to mujaheddin in Afghanistan's central and northern provinces.

Several resistance sources said the base had been captured over the weekend, but the reports were denied today by Jalaluddin Haqqani, the base's commander, speaking by telephone from Miramshah.

While military analysts believe the Soviets and Afghan troops could not hold Jawar, they suggested that the base's destruction would be an important symbolic defeat for the rebels and a significant disruption of their resupply effort. Soviet-led forces tried and failed to destroy Jawar in a similar offensive against the Paktia supply routes last August and September.

"The mujaheddin have knocked down a few helicopters, but the Soviets are using more than a hundred of them, and we really have no defense," said resistance commander Amin Wardak, whose fighters use the Paktia trails to supply their own bases in Wardak province, to the west of Paktia.

Sources in Washington said last month that the administration had decided to supply Stinger shoulder-fired missiles to the rebels, who depend on largely ineffective heavy machine guns and a few Soviet-made SA7 missiles to defend against helicopter gunships and jets.

It was unclear when the decision was to be implemented, although the resistance could be supplied quickly if, as some military observers here suggested, the missiles are already held by the Pakistani military.

Haqqani and other resistance sources were unanimous in saying that no Stingers have yet reached the battlefield. He asserted, however, that his forces have shot down as many as 11 Soviet and government aircraft during the offensive.

Military observers here argued that training difficulties will likely prevent any quick deployment of Stingers in Afghanistan. One western military analyst in Islamabad said recently that regular U.S. troops who already have had basic training require many days' special instruction to learn the fundamental use of the Stinger.

Afghan resistance fighters, most of whom are illiterate and lack basic math skills, would require long training periods on the sophisticated Stinger, it was argued. Diplomats here said last week that Pakistani officers, many of whom speak one of the major Afghan languages, would be the logical choice to train the rebels on the U.S. missiles. But, they added, Pakistan would be reluctant to commit itself to any training program, for fear of provoking the anger of the Soviet Union.

The Soviet and Afghan government forces launched both of their current offensives, in Paktia and Kandahar, on April 4. Each battle appears to be among the largest of the past year.

Elite commandos of the Afghan Army opened the Paktia offensive, but suffered heavy casualties during the first two weeks and were reinforced several days ago by about 2,000 Soviet troops, according to the Afghan Islamic Press, a frequently reliable, proresistance news agency based in Peshawar.

Resistance commanders claim to have killed or captured several hundred of the government forces, while acknowledging 200 casualties of their own. Kabul radio claimed that 700 guerrillas had been killed in one day's fighting.

At least three times in the past week, Pakistan has protested formally to the Afghan Embassy here over what it said has been a spate of cross-border shelling and bombing incidents, which the Pakistani government said killed at least six and injured about a dozen Pakistanis and Afghan refugees. Most of the incursions were reported adjacent to the fighting around Jawar.

Hospitals in the Pakistani town of Quetta were reported overflowing with casualties, including many civilians, from the fighting in Kandahar, 125 miles to the northwest. Earlier this month, Soviet and Afghan government troops with hundreds of armored vehicles were reported to have surrounded the city, trapping more than 1,000 mujaheddin inside.

Although the Soviet and Afghan troops allowed women and children to leave the city, Afghan sources in Quetta said, many civilians remained and were caught in the intense fighting, said to include hand-to-hand combat for control of some neighborhoods.

Reports from Quetta over the weekend said about 170 rebels had died in five days of fighting last week. There was no indication of Soviet and Afghan casualties, which also were thought to be heavy.

Western analysts here believe the Soviet and government offensive in Kandahar may have been provoked by an attempt by the resistance to improve their coordination in the province.