GARDEZ, AFGHANISTAN -- Just as the Afghan rebels have dispelled doubts about their military effectiveness, they are facing questions regarding their ability to manage conquered areas and acquiring a reputation that could hamper the continued success of their spring offensive.

After two years of defeats and feeble attacks, the Afghan rebels, in the past eight weeks, have captured Khost, 60 miles southeast of here, and Kwaji Ghar, the capital of the northern province of Takhar, and have begun attacks on the southern city of Kandahar. And in the hills and villages outside Gardez, about 70 miles south of Kabul, the rebels, or mujaheddin, are preparing for what they hope will be a repeat of their successful siege at Khost, where the local garrison surrendered.

But analysts agree that the failure of the mujaheddin to prevent violence in Khost after their takeover could bode ill for their efforts to force a surrender here and in other towns. "If they are going to succeed, they're going to have to do better than they did at Khost," said a high-ranking Western diplomat in Pakistan.

Khost, which had a prewar population of 15,000, lies ruined and devoid of civilian life. The wide, tree-lined streets of the once-thriving agricultural and commercial center are littered with debris and the remains of the dead. In what had been the bazaar, human bones, gnawed clean by dogs, lie among heaps of trash from looted shops.

Khost residents have fled, fearing both the conquering rebel forces and the aerial bombardment and Scud missile attacks unleashed by the Kabul regime after the local garrison capitulated.

Commander Jalaludin Haggani, 55, the Muslim cleric who was the principal architect of the victory at Khost, issued a proclamation of amnesty for civilians and regular army troops a week before the town fell. The negotiated surrender concluded the longest siege in the 13-year war.

Accounts from foreign aid workers operating in the area indicate that for the most part, Haggani kept his word. All reports agree that regular army troops were disarmed and either released or taken as prison laborers.

Haggani could not, however, control Waziri and Mahsud tribesmen from both sides of the border with Pakistan who swept into the city in the wake of rebel forces. The heavily armed tribesmen, who numbered more than a thousand, looted the town completely, destroying what could not be carried off, and have taken de facto control of the town. Members of Haggani's organization have insisted that Western journalists visiting Khost be acompanied by armed bodyguards.

The rebels say the urgency of securing captured arms and ammunition at Khost precluded an effort to contain the tribesmen.

After the Soviet troop withdrawal was completed in February 1989, the Muslim insurgents repeatedly disappointed their supporters. Their disastrous defeat at the battle of Jalalabad in the spring of 1989 was followed by only weak attempts to seize small government positions. Moreover, continuous, bitter infighting among rival resistance groups left many analysts doubtful that the Soviet-backed Kabul regime would be toppled.

But last year, in a bid for greater unity and military success, the most prominent rebel commanders formed the Commanders' Shura, or council, which is rapidly eclipsing the authority of the Peshawar-based political parties. "The Commanders' Shura is the broadest-based organization since the beginning of the war," according to the Western diplomat in Pakistan.

"This year is very different," the diplomat said. "In '89 and '90 you had a holdover from the anti-Soviet movement. Now you have the beginnings of a real nationalist movement focused on removing {Afghan President} Najibullah. Whoever captures Afghan nationalism will win this second war."

In an interview last fall, Ahmed Shah Massoud, the commander responsible for the capture this spring of Kwaji Ghar, expressed the same sentiment, "The Soviets left too soon. We didn't have time to make the shift from regional resistance to a national revolutionary movement. That is the direction we must take now."

Observers agree that recent mujaheddin gains are significant. A Western official in Peshawar with close ties to resistance leaders said, "To put it in baseball terms, the mujaheddin have started off their season two for two."