At least 50 Syrian rebels were killed on Sunday as the country’s “second civil war” between moderate and jihadist rebels continued, AFP reported.
The latest deaths came one day after several groups of Syrian rebels said they had united for the purpose of launching a new “revolution” against the Al-Qaeda-affiliated the Islamic State of Iraq and the Levant (ISIL).
The Syrian Observatory for Human Rights told AFP the rebels were killed in fighting, car bombings and summary executions by forces loyal to the jihadist group.
Sunday's deaths came on day three of fighting that began when rebels, including Islamists, attacked checkpoints and bases manned by ISIL, which is accused of horrific abuses in areas where its fighters operate.
ISIL has been taking key roads and checkpoints from its rivals, and some opponents of President Bashar Al-Assad have even accused it of serving regime interests.
At least nine fighters from the jihadist group were also killed on Sunday, the Britain-based Observatory said.
Most of the non-ISIL fatalities were in the northern provinces of Aleppo and Idlib, where the fighting began on Friday.
Among the dead were "seven who were summarily executed by ISIL", AFP quoted the group as saying.
Clashes also erupted in Raqa and Hama provinces on Sunday, in a sign the fighting was spreading.
Scores of fighters on both sides have been killed in the past three days, the Observatory says.
"The rebels have made significant gains (since Friday) in the west of Aleppo province, in the north of Idlib and east of Maaret al-Numan (Idlib)," said Observatory director Rami Abdel Rahman.
The Observatory also said ISIL fighters were laying down their arms and withdrawing from some areas, without elaborating.
The Islamic Front, the country's biggest rebel alliance, issued a strong warning to the jihadists.
"We fight against whoever attacks us and whoever pushes us to battle, whether they are Syrian or foreign," said the alliance grouping tens of thousands of anti-Assad rebels.
Meanwhile, an ISIL statement similarly warned the rebels to stop pressuring them, or they would withdraw from the front lines in Aleppo city and let in Assad's forces.
The Islamic Front on Sunday said it was "thankful to the foreigners who came to help us" but "we will not accept any group that claims to be a state".
ISIL is one of several jihadist rebel groups operating in Syria and which at one point split off from the Western-backed Syrian National Council opposition force, declaring Aleppo to be an independent Islamist state.
In addition to its involvement in the Syrian war, the Al-Qaeda affiliate seized the city of Fallujah in neighboring Iraq on Saturday, and claimed a suicide bomb attack in a Beirut stronghold of Hezbollah, the movement fighting alongside Assad's forces.