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An explosion in Syria killed at least 28 high-ranking members of an Islamist insurgent group that is fighting the Islamic State, Reuters reported Tuesday, citing a group that monitors the war raging there.

The blast took out the leader of Ahrar al-Sham, Hassan Aboud, the group confirmed on its Twitter feed, according to Reuters. The hardline group is a member of the Islamic Front alliance fighting ISIS and had at one time been considered the strongest insurgent group in Syria.

Al-Jazeera reported that the attack was a suicide bomb targeting a high-level meeting of rebel groups in Idlib province. It cited a death toll of between 20 and 45.

Foreign Affairs noted in January that Ahrar al-Sham had also been linked to al Qaeda, one of several factors complicating the debate over whether the U.S. should aid them in their fight against ISIS.

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