ISTANBUL — Al Qaeda’s central leadership has officially cut ties with a powerful jihadist group that has flourished in the chaos of the civil war in Syria and that rushed to build an Islamic state on its own terms, antagonizing the wider rebel movement.

The animosity between the group, the Islamic State of Iraq and Syria, or ISIS, and other rebel groups has fueled the deadliest infighting yet between the foes of President Bashar al-Assad and has sapped their campaign to depose him.

Though the isolation of the group could lead to greater unity among other rebel forces, it is unlikely to assuage fears in the United States and elsewhere about the increasing power of extremists in Syria.

The break between Al Qaeda and the Islamic State of Iraq and Syria, announced late Sunday on jihadist websites, served both sides, said William McCants, a scholar of militant Islam at the Brookings Institution. Al Qaeda cut ties with a group that was besmirching the Qaeda name among other militants, while the Islamic State of Iraq and Syria bolstered its image as a force to reckon with.