Al-Qaeda-linked fighters killed at least 50 rival rebels Sunday in a new front in Syria's war, a watchdog said, as an Islamist group issued a strong warning to the jihadists.
The Syrian Observatory for Human Rights said the rebels were killed in fighting, car bombings and summary executions by forces loyal to the Islamic State of Iraq and the Levant.
ISIL moved into Syria in late spring 2013, two years after the uprising against President Bashar al-Assad began.
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 Assad opponents have even accused it of serving regime interests.
At least nine ISIL fighters 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".
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.
In Atareb, also in Aleppo province, rebels raised the green, white, black and red flag of the opposition, and brought down the black flag flown by ISIL.
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.
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Revolution being 'cleaned up'
The Islamic Front, the Syrian Revolutionaries Front and the nascent Army of Mujahedeen are all battling ISIL.
"What's happening is that the ranks of the revolution are being cleaned up," one Islamic Front member told AFP.
On Saturday, an ISIL statement warning 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".
Analysts say a key complaint against ISIL among rebels, including Islamists, is that the jihadists refuse to operate under the broader opposition.
Instead, it commands its own institutions and rejects cooperation with other rebel groups.
Anti-ISIL protesters demonstrated in Aleppo and Raqa for a third day on Sunday, activists said.
They are also calling for countrywide protests Friday under the slogan: "Day of rage against Al-Qaeda and Assad".
The slogan echoes the spirit characteristic of the pro-democracy protests that began in March 2011 before becoming a bloody civil war.
In a separate development, 10 soldiers were shot dead by Al-Nusra Front, another jihadist group, days after being captured in Aleppo's Kindi hospital, which the army had turned into a base.
Reflecting the complexities of Syria's war, Al-Nusra has also been involved in some of the fighting against ISIL, despite both groups having their roots in Al-Qaeda in Iraq.
In other developments, Riad Hijab, a former Syrian premier who defected in 2012, is a frontrunner along with current chief Ahmad Jarba to head the main opposition National Coalition in six-monthly elections being held in Istanbul, a coalition member said.
The Coalition is also expected to take a final decision on whether to attend proposed peace talks in Switzerland slated for January 22.
And Turkish photographer Bunyamin Aygun, kidnapped by radical Islamists in Syria while on assignment for the newspaper Milliyet, has been freed, the Anatolia news agency reported.