Since the summer of 2014, Islamist rebels and ISIS fighters have clashed in northern Aleppo with the front line moving back and forth. However, with the Syrian Government and Kurdish militants advancing all across Aleppo governorate, this has gradually caused Islamist fighters on both sides to halt their hostilities with each other.
Effectively, the front line situation in northern Aleppo has reached a stalemate with both sides de-escalating and gradually pulling out their fighters as to cover other fragile front lines. Rebels and the Islamic State have somewhat common enemies (the Syrian Army and YPG/SDF) and largely similar interests (implementing religious laws). As such, they have put aside their differences for their greater concerns and as to strengthen other areas.
Less than 3 years ago, ISIS was considered part of the opposition to the Syrian government. From mid-2012 until the end of 2013 they had both shoulder-by-shoulder succeeded in seizing Managh Airbase from hopelessly entrenched government troops during a joint offensive which included ISIS, Jabhat al-Nusra, Jaish al-Islam and the Free Syrian Army.
In November of 2015, rebel forces and ISIS carried out simultaneous offensives from each flank on the Syrian Arab Army near Ithriya village along the government supply line to Aleppo city but were later repelled. Once again, just two days ago, rebel forces and ISIS carried out another joint offensive and even managed to capture and hold the strategic village of Khanasser during the process.
In the summer of 2014, ISIS became too powerful and left the opposition to pursue its own ambitions and proclaim a regional caliphate. For now, it appears that both sides (ISIS and rebels forces) have been weakened inside Aleppo province and as such must seek to shift the balance of power by putting an end to their war with each other.