The difference between the two Qaeda affiliates has more to do with their approach than with their way of thinking, analysts say. “Their ideologies are very much the same, but Nusra is really embedding itself in the Islamic landscape, working with other groups and trying to compromise, while ISIS has been doing the opposite, which is why they have no more friends,” said Aron Lund, a researcher who edits a website on the Syria conflict for the Carnegie Endowment for International Peace.

On Tuesday, in an audio recording released online, the Nusra Front’s leader called for a cease-fire and the creation of an Islamic court to mediate disputes. In a second recording released Tuesday and attributed to a spokesman for the Islamic State of Iraq and Syria, the group threatened to “crush” its enemies.

Rebel anger had been building for months, but most hesitated to challenge the group.

“The rebels avoided confronting ISIS in the beginning because they didn’t want to be distracted from fighting the regime,” the activist Abdul-Rahman Ismael said by Skype from Aleppo. “They hoped that ISIS would help topple the regime but found otherwise, so it became necessary to fight ISIS before fighting the regime.”

The recent infighting here is attributed by many to the death of a rebel doctor, Hussein Suleiman, who was detained by the group and returned to his colleagues last week with bullet holes in his shoulder and the top of his head missing. Photos and videos of the dead doctor spread on social media, fueling the outrage.

Subsequent episodes further outraged the rebels: One of their leaders, detained by the Islamic State of Iraq and Syria, was found dead on the side of a road, and fighters from the group seized a former Syrian Army base that rebels had been using since last year. The fighting has produced grim scenes reminiscent of government killings of opposition activists earlier in the war.

The Syrian Observatory for Human Rights said Wednesday that at least 385 people had been killed in five days of rebel infighting, including 56 civilians. The group, which tracks the conflict from Britain through a network of contacts in Syria, also said rebels had killed more than 40 fighters from the Islamic State of Iraq and Syria in Idlib Province.

Opposition activists who have compared the group’s heavy-handed tactics to those of Mr. Assad’s government were glad to see it pushed from Aleppo. One of them, who goes by the nom de guerre of Abu Fatih, said the group’s fighters accused him and his colleagues of being heretics, evicted them from their office and barred them from smoking in the street.

“Now my neighborhood has been liberated twice,” he said. “Once from the regime and the second time from ISIS.”