The warning came yesterday as a cease-fire ended fighting near the Turkish border between the mainstream rebels and fighters belonging to the al-Qaida offshoot known as the Islamic State in Iraq and the Levant. During the battle, the jihadis overran the town of Azaz.
As the cease-fire took hold, al-Qaida militants fought heavy street battles against Kurdish gunmen in northern Syria.
The infighting was some of the worst in recent months between forces seeking to bring down President Bashar Assad, and it threatened to further fragment an opposition movement outgunned by the regime.
"ISIL no longer fights the Assad regime. Rather, it is strengthening its positions in liberated areas at the expense of the safety of civilians," the statement said. "ISIL is inflicting on the people the same suppression of the Baath party and the Assad regime."
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Al-Qaida-linked fighters in Syria have been some of the most effective forces on the battlefield, fighting alongside the rebels' Free Syrian Army against government forces. But the two factions have turned their guns on each other, and turf wars and retaliatory killings have evolved into ferocious battles that have effectively become a war within a war in northern and eastern Syria, leaving hundreds dead on both sides.
The deal calls for setting up a checkpoint between the two sides. They also agreed to take disputes before an Islamic council that would soon be established.
The fighting in Azaz and the prospect of al-Qaida militants so close to the frontier prompted Turkey to close a nearby border crossing.