Nearly 50 fighters have been killed in three days of clashes between jihadists and mainstream rebels in Aleppo, the Syrian Observatory for Human Rights said on Saturday.
It said the fighting erupted on Thursday between militants of the Al-Qaeda-linked Islamic State of Iraq and the Levant (ISIL) and a battalion linked to the Arab- and Western-backed Free Syrian Army (FSA), both of which are fighting to topple President Bashar al-Assad.
"At least 30 fighters from the Ababil Brigade and 14 from ISIL have been killed in combat, and that toll could rise further," the Observatory's Rami Abdel Rahman told AFP.
He said the clashes occurred in several districts of Syria's second city and former commercial hub, and that ISIL, which expounds an extreme form of Islam, made gains in three sectors.
The ISIL militants have now consolidated their presence in Aleppo, which has been basically split into the rebel-held east and pro-regime west since July last year.
Although both claim the ouster of Assad as a common aim, jihadists and the mainstream rebels have come to blows many times in recent months.