The UK-based Syrian Observatory for Human Rights says at least 482 people have been killed in the fighting that has pitted a loose alliance of ultra-conservative rebel brigades and more moderate factions against the fighters of the al Qaeda-linked Islamic State of Iraq and the Levant.
The toll includes 240 rebels, 157 fighters from the Islamic State and 85 civilians.
Fighting has spread across four provinces in opposition-held parts of northern Syria since it began last week.
The al Qaeda-linked group has alienated other factions over the past several months by using brutal tactics to implement its strict interpretation of Islamic law, and by kidnapping and killing of opponents.
Meanwhile, Syrian soldiers have killed dozens of rebels who tried to break the military's siege on opposition-held areas of the central city of Homs.
At least 37 rebels were killed in the Matahen area of Homs, the official SANA news agency said. The death toll was put at 45 opposition fighters by the Observatory which said they were killed in heavy fighting late on Wednesday and early Thursday.
It had no information on government casualties. For months, forces loyal to Syrian President Bashar Assad have imposed a siege on opposition-held areas of Homs.