Afghan resistance groups have succeeded in reopening some supply routes that were broken by a Soviet offensive last month, but guerrilla sources here said last week that the Soviets are reinforcing major garrisons in eastern Afghanistan and that a major Soviet drive is believed to be imminent.

The resistance fighters, or mujaheddin, say they are recovering from the bloody Soviet-backed offensive against Jawar, their major base in Paktia Province, in which hundreds of their forces were killed and wounded. They say they have now reentered Jawar, which the Soviet captured but left shortly thereafter.

Independent analysts agreed that the mujaheddin also inflicted heavy casualties on the attackers, especially on the initial force of about 400 Afghan government troops dropped near the base by helicopter.

But the heavy losses suffered by the guerrillas have revived an internal debate over their military strategy and, in the view of some resistance officials, will lead to a change in tactics. "Perhaps they learned a good lesson not to try to keep such a big base and become immobile," said Sayed Majrooh, Afghan editor of a monthly news bulletin on the war.

Several resistance sources said the mujaheddin are now reestablishing their base at Jawar, near the Pakistani border, after it was partly destroyed in late April by the Soviets. Afghan government soldiers taken prisoner in the battle are being forced to clear land mines left by the Soviets around the base, and several have been killed or wounded, said Mohammed Yaqub Sharafat, of AIP, the resistance-linked Afghan Islamic press agency.

Sharafat and other sources said the current fighting appears heaviest to the north, where smaller guerrilla camps are clustered near the border, southwest of the Pakistani town of Parachinar.

Resistance officials said in interviews last week that many of the supply trails passing near Jawar were open again, but that a large Soviet force remained nearby, at the garrison town of Khowst, to the northeast. Resistance leaders said that about 2,000 Soviet troops and large amounts of heavy artillery had been flown to Khowst last month, to help replace Afghan government forces in the final attack on Jawar.

"With all the Soviet reinforcements still at Khowst, we think perhaps they will start a new offensive, maybe to open the road to Gardez," to the west, Sharafat said. In repeated offensives, the Soviets and Afghan government have failed to open the road or lift the guerrilla siege of Khowst.

Many Afghans said they expected the Soviets to launch a major operation on or after the beginning yesterday of the holy month of Ramadan, in which Moslems fast during daylight, although participants in a jihad, or holy war, are exempt.

According to a diplomatic analyst, the Soviets appear to have reinforced Khowst and other garrisons in eastern Afghanistan during the winter before launching offensives from February through April in Nangarhar and Paktia provinces. The offensives and the reinforcements at Khowst have strengthened expectations here of a general escalation this year in the Soviet campaign to close the network of mountain trails passing through the region. The trails link resistance bases in Pakistan with fighting units in the center and north of the country.

That campaign has included political efforts by the government in Kabul to turn Pashtun tribes in the region against the mujaheddin. Those political efforts, which have been the specialty of the newly selected Afghan leader, Najibullah, have combined with the military campaigns to close off some of the easiest trails through the region, forcing the mujaheddin to negotiate more difficult terrain and higher passes.

Along with the southern city of Kandahar, the eastern provinces have apparently seen the heaviest fighting of recent months -- although such judgments are made difficult by the fact that news flows more easily from the Pakistani border region than from other parts of Afghanistan.

"There has been heavy military pressure on the border areas, including in Kunar" Province, said Hashmatullah Mojaddidi, a leader of the Afghan National Liberation Front, one of the seven main resistance parties.

Mojaddidi agreed with several other resistance officials that last month's offensive against Jawar was the most difficult of several campaigns the Soviets and Afghan government forces have mounted in the area in recent years. The mujaheddin said they lost 125 killed and more than 200 wounded, although western observers said the figures probably were higher.

Several resistance officials agreed that the mujaheddin may have tried to hold the Jawar base too long, thus exposing themselves to heavy air and artillery strikes against which they had no defense.

Jawar, an elaborate base dug into the sides of a canyon and equipped with repair shops for captured tanks and other heavy equipment, assumed, over time, a symbolic importance for the resistance. Several groups of mujaheddin insistently denied its fall last month until Soviet forces withdrew and they announced its "recapture."

"After holding it for seven years, the mujaheddin began thinking Jawar was impregnable," said Mohammed Es'Haq, a political officer of the Jamiat-i-Islami party. "But no matter how strong we are, we are still guerrillas, and we should not try to hold territory or fixed bases like this. Now, I think, we will remain more mobile," he said.

Abdul Haq, a commander of the Hezb-i-Islami faction led by Younis Khales, argued that the Jawar battle had not been a clear defeat for the mujaheddin. "I am against this idea that we should build big camps, but Jawar did do one good thing for us," he said. "It pulled the enemy out into the hills, where we would rather fight him, rather than inside his bases."

Abdul Haq argued that the Soviets had meant to take over the base permanently but had abandoned the plan in the face of heavy resistance, but others were unsure whether the Soviets had meant to stay in the area.

Whatever the damage inflicted by the mujaheddin, no one suggested that they should repeat the experience, and many argued that the base should be rebuilt as a smaller facility.

Although Sayed Majrooh reported that four jets and five helicopters had been shot down in the three-week battle, the mujaheddin said they continue to lack air defenses. They expressed exasperation over reports that the Reagan administration is supplying them with sophisticated Stinger shoulder-fired antiaircraft missiles.

"It's another boast, another rumor, another betrayal," said Es'Haq, "to announce this in the newspapers only provokes the Soviets and gives them a pretext to escalate the fighting."

Stingers are known to be in the inventory of the Pakistani armed forces, and military analysts here have suggested that they could have been transferred quickly to the mujaheddin. But several mujaheddin commanders said they had heard of no Stinger deliveries, and it remained uncertain how the guerrillas would receive the extensive training necessary to aim the missiles properly.