Clashes between Syrian rebel and fighters from an al-Qaeda-linked group have killed nearly 500 people in northern Syria over the past week, according to a monitoring group .
The UK-based Syrian Observatory for Human Rights says that at least 482 people have been killed in the fighting that has pitted a loose alliance of rebel brigades with different ideologies against the fighters of the Islamic State of Iraq and the Levant (ISIL).
The toll includes 240 rebels, 157 ISIL fighters and 85 civilians, the group said on Friday.
The fighting has spread across four provinces in opposition-held parts of northern Syria since it began last Friday, causing large-scale supply shortages.
Reports say ISIL has alienated other factions over the past several months by using harsh tactics to implement its strict interpretation of Islamic law, and by its kidnapping and killing of opponents.
However, ISIL fighters made progress on Friday in their campaign to retake al-Raqqa from rebel groups.
The region came under ISIL's sway a few months after President Bashar al-Assad's regime lost control in March 2013 of al-Raqqa, the provincial capital.
"In al-Raqqa, ISIL is winning because its supply routes [to Iraq] are open there," Alaaeddine, an activist in the country's north, said.
ISIL, which includes mostly foreign fighters among its ranks, has its roots in al-Qaeda's Iraqi affiliate, and first appeared in the Syrian conflict in late spring last year.
However, in Aleppo and Idlib, ISIL is on the defensive, activists said.
"In Idlib there are practically no ISIL bases left, as is the case in Aleppo city and the west of the province," Alaaeddine said.
While rebels initially welcomed self-styled "holy warriors" in Syria, Alaaeddine said ISIL's abuses and quest for hegemony has turned "90 percent of people" in opposition areas against it.
In the central Hama province, Free Syrian Army rebels captured fighters from the ISIL after storming the group's base on Thursday.
The latest fighting between the rebels and the ISIL came a day after Assad's troops killed dozens of rebel fighters in the central city of Homs, both state media and activists reported.
The official SANA news agency said on Friday the military killed at least 37 rebels in the Matahen area of Homs.
Activists put the death toll at 45 people and said the fighters were killed as they tried to break a siege imposed by Assad forces since June 2012.