The announcement, reported by the UK-based Syrian Observatory for Human Rights (SOHR), came as the Al-Qaeda-inspired Islamic State in Iraq and Syria (ISIS) overran much of neighboring Iraq’s second-largest city, Mosul.
Details of the new offensive were not immediately known. But one of the groups in the coalition, the Islamic Front, claimed that its fighters recently captured four villages from ISIS and killed 17 of its fighters.
The infighting is part of broader rebel-on-rebel clashes that have raged across opposition-held northern Syria since early January.
The new coalition, called the “Operations Room for the people of the Levant” also includes Kurdish groups in Syria, which traditionally have focused on defending their own ethnic areas from other rebel groups, said Rami Abdurrahman of the SOHR.
ISIS has strongholds in the northern province of Aleppo in the towns of Manbij and Al-Bab. It also controls a swath of territory from the northeastern city of Raqqa to the eastern province of Deir Ezzor, where it has also been battling for control with rival rebel groups.
On Tuesday, the Observatory said the fighting in Deir Ezzor alone has killed 634 since April 30. It said the tally includes 39 civilians, 354 rebel fighters, including many from the Al-Qaeda-linked Nusra Front, and 241 gunmen from ISIS.