Clashes between rival rebel factions left at least 44 fighters dead in battles to control neighbourhoods in the city of Aleppo, according to an activist group.
The three days of fighting was between al-Qaeda's Islamic State of Iraq and Sham (ISIS) and a rival group formerly known as Ghurabaa al-Sham.
The British-based Syrian Observatory for Human Rights (SOHR) said on Saturday 14 of the dead belonged to ISIS, which was able to control three neighbourhoods in Aleppo.
Rebel groups have become increasingly fractured, and enabling fighters linked to al-Qaeda to assume prominent roles in battle.
In an audio message on Friday, the leader of al-Qaeda, Ayman al-Zawahri, urged fighters in Syria to "rise above organisational loyalties and party partisanship'' to unite and set up an Islamic state.
He suggested he would not impose unity, saying that "what you agree upon will also be our choice".
Two al-Qaeda-linked groups have emerged; Jabhat al-Nusra and ISIS. The first is commanded by a Syrian, the second by an Iraqi, but both are believed to be loyal to Zawahri.
Meanwhile, a child was killed and several people injured after two mortar shells hit Syria's capital near a hotel where international chemical weapons inspectors and United Nations staff are staying, state media and a hotel guest said on Saturday.
The blasts in the upscale Abu Roumaneh area of Damascus killed an eight-year-old girl and wounded 11 other people, the SANA news agency said.
The girl was in her family car near the school when she was killed, the SOHR said.
The blasts struck some 300 meters away from the Four Seasons Hotel where the chemical weapons inspectors and UN staff are staying.
A UN employee staying there said it did not appear that the hotel was affected by the explosions.
Syrian rebels routinely fire mortar shells from the outskirts of Damascus at city neighbourhoods controlled by forces loyal to President Bashar Assad.
Inspectors from the Organisation for the Prohibition of Chemical Weapons and UN staff have been in Syria for the past two weeks to destroy the country's chemical weapons stockpile.
The OPCW inspectors have so far visited three sites linked to Syria's chemical weapons program, though the agency has not provided details.