Syria’s Islamic Front, the country’s biggest rebel alliance, issued Sunday a strong warning to Al Qaeda jihadists with the Islamic State of Iraq and Greater Syria (ISIS, also known as ISIL) , three days after a new front made up of local insurgents emerged against them.
“We fight against whoever attacks us and whoever pushes us to battle, whether they are Syrian or foreign,” said the Front, a powerful alliance that groups tens of thousands of mostly conservative Islamist fighters seeking to topple President Bashar Assad.
The latest clashes broke out Friday after residents accused ISIS members of killing a doctor in Syria’s northern province of Aleppo.
The Islamic Front issued a statement ordering ISIS to hand over the doctor’s killers so they could be put on trial. Clashes later erupted between the groups.
The focus of combat has been in opposition areas in Aleppo and Idlib provinces, but Sunday spread to Hama and Raqqa provinces.
Scores of fighters have been killed on both sides. ISIS killed at least 50 rival rebels Sunday, according to the Britain-based Syrian Observatory or Human Rights, citing insurgents and medics, including seven “who were summarily executed by ISIS” in Harem, in Idlib. 
Some of the heaviest fighting Sunday took place in the town of Manbij in Aleppo province, where rebels seized an ISIS compound, activists said. The Observatory said ISIS fighters used car bombs, a tactic usually reserved for attacking government forces, for the first time to defend its territory. 
Clashes also erupted in the town of Tabqa in the northern province of Raqqa, where ISIS is most dominant, said Observatory director Rami Abdel-Rahman
ISIS fighters ceded ground near the Turkish border to rival Islamists Sunday, activists said, in what seemed to be a tactical withdrawal.
Syrian opposition activists said ISIS had pulled back Sunday from strong points including Dana and Atme in Idlib province and that fighters from the Nusra Front and Ahrar al-Sham, with the Islamic Front, had moved in.
“The Islamic State is pulling out without a fight. Its fighters are taking their weapons and heavy guns,” activist Firas Ahmad said. He added that the ISIS fighters headed in the direction of Aleppo, where Assad’s troops have stepped up pressure on rebel forces who captured the city 18 months ago.
Another activist, Abdallah Al Sheikh, said that some Syrian ISIS fighters had stayed in place but switched allegiance to the Nusra Front. Nusra’s commanders are mostly Syrian rather than foreign and it coordinates with the Islamic Front, but both ISIS and Nusra have their roots in Al-Qaeda in Iraq.
ISIS has been accused of horrific abuses in areas where it operates, and also of seeking hegemony by taking key roads and checkpoints from its rivals.
“In our charter ... we said we are grateful and thankful to the foreigners who came to help us” in the war against Assad’s troops, the Islamic Front said in Sunday’s statement.
But “we will not accept any group that claims to be a state.” Protesters, meanwhile, took to the streets of Aleppo and Raqqa for a third consecutive day chanting slogans against ISIS, activists said.
Activists are calling for countrywide protests next Friday under the slogan: “Day of rage against Al-Qaeda and Assad.”
The slogan echoes the spirit of the pro-democracy protests that began in March 2011 before becoming a bloody civil war.
In Atareb, also in Aleppo province, rebels raised the green, white, black and red flag of the opposition, and brought down the flag flown by ISIS.
“What’s happening is that the ranks of the revolution are being cleaned up,” a member of the Islamic Front told AFP on condition of anonymity.
Analysts say a key complaint against ISIS among rebels, including Islamists, is that its jihadists refuse to operate within the broader opposition dynamic. Instead, it commands its own institutions and rejects cooperation with other rebel groups.
The Islamic Front’s statement comes a day after ISIS distributed an audio statement warning rebels to stop pressuring it, or that it would withdraw from the front lines in Aleppo and let in Assad’s forces.
ISIS also accused its rivals of waging a “media war” against it, and of “stabbing [it] in the back.”
Despite the days of skirmishes, opposition supporters and diplomats said that a broad alliance involving these groups seemed to be holding in the desert east of the country. “There is certainly competition between ISIS and the other Islamist militants, but it does not appear there is full-scale confrontation,” a Middle Eastern diplomat said.
Separately, 10 regime troops were shot dead by Nusra days after being captured in Aleppo’s hospital, which the army turned into a base.