U.S.-backed rebels & al Qaeda are "brothers of ISIS in Islam, and will not accept a non-Islamic or non-Shariah-based constitution.”

McClatchy

"...The group affiliated with the Free Syrian Army that brokered Thursday night’s cease-fire, Liwa al Tawheed, has long worked closely with the Islamic State of Iraq and Sham and its al Qaida-linked ally, the Nusra Front, and wants to avoid an escalation, said Aymenn Tamimi, a fellow at the Philadelphia-based Middle East Forum who’s a doctoral candidate at Oxford University.“I don’t see the Azaz clashes as evidence of an imminent ‘FSA vs. ISIS’ war,” he said by email. “Notably, Liwa al Tawheed’s commanders still value ISIS as a military asset and were accordingly pushing for a compromise in Azaz, even as many on-the-ground supporters and lower-rank fighters are much more suspicious of ISIS.”
Tamimi also said that another FSA-affiliated group, the Farouq Brigades, which battled Islamic State fighters last week in the town of al Bab in Aleppo province, was likely to want to preserve its contacts with Islamic State fighters. Farouq leaders, he said, “stress they are brothers of ISIS in Islam, and will not accept a non-Islamic or non-Shariah-based constitution.”..."