BEIRUT—Rival Syrian rebels on Wednesday ousted a branch of al Qaeda from its base in Aleppo, whose members executed dozens of Syrian prisoners as they left, leaving the fate of Western hostages they held uncertain, activists said.

Hundreds of fighters belonging to the Islamic State of Iraq and al-Sham, an arm of the extremist Islamist movement, were also driven out of towns across northern Syria this week after their rivals began to rise up against them last week.

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