Nearly 1,400 people have been killed in Syria since clashes between rebel forces and the Islamic State of Iraq and the Levant, an extremist group composed of mostly foreign fighters, erupted this month, the Syrian Observatory for Human Rights said.
"The number of people killed in fighting between the Islamic State of Iraq and the Levant and Islamist and rebel forces since January 3 has risen to 1,395," SOHR said.
The UK-based rights group said the figure included 760 moderate rebels, 426 ISIL fighters, 190 civilians and 19 others whose identities have not been established.
Clashes between rebels and foreign fighters from ISIL erupted early in January after months of rising tensions.
While opposition fighters initially welcomed foreign fighters to the battle against government forces, ISIL has been accused of a string of abuses against civilians and rival rebel groups.
Among the abuses that sparked the fierce clashes was the kidnap, torture and execution by the group of a doctor from a powerful rebel brigade.
The all-out fight has seen ISIL lose territory in Idlib and Aleppo provinces, but it has consolidated its hold over Raqqa city, the only provincial capital to fall from regime control.