An alliance of Islamist and other rebel factions are battling fighters from the Islamic State in Iraq and the Levant (ISIL) across north-western Syria in apparently coordinated strikes against the powerful al-Qaeda-linked group.
Activists said dozens of fighters had been killed in the clashes, which started yesterday and may have been provoked by increasing resentment against the radical ISIL fighters, many of them foreign jihadis.
One group of fighters battling the ISIL was the newly formed Mujahideen Army, an alliance of eight brigades who accused the al-Qaeda affiliate of hijacking their struggle to topple Syrian President Bashar al-Assad.
It said ISIL fighters were "undermining stability and security in liberated areas" through theft, kidnapping and trying to impose their own brand of Islam.
It has vowed to fight them until ISIL was disbanded or driven out of Syria.
The infighting amongst Mr Assad's opponents has strengthened his hand ahead of planned peace talks in Geneva on 22 January. Mr Assad, backed by Shia fighters from Iraq and Lebanon's Hezbollah militia, has pushed back rebels around Damascus and in central Syria, and faces little pressure to make concessions.
Fighters from the Islamic Front, made up of several Islamist brigades which have been close with ISIL in the past, were engaged in heavy clashes with the group in northern Aleppo province, according to the Syrian Observatory for Human Rights.
The Observatory, a monitoring group based in Britain, said at least 60 people had been killed in fighting which it described as a major challenge to ISIL's control in Aleppo and neighbouring Idlib province.
The ISIL and another al Qaeda affiliate, the Nusra Front, together with Islamist fighters from the Islamic Front, have eclipsed the Free Syrian Army which Western powers had hoped to build into a moderate force capable of toppling Assad.
That impotence was highlighted in November when the FSA's military command lost control of a military base and main weapons depot close to the Turkish border.
Mr Assad's main political opponents in exile, the National Coalition, sought to portray the clashes as a counter assault by the FSA against ISIL's "authoritarian oppression"
"The Syrian people clearly have rejected al Qaeda's attempts to establish a presence in the liberated territories," coalition member Monzer Akbik said.
"The solution to fighting extremism inSyria is to strengthen the Free Syrian Army at this critical juncture".
The coalition said the fighting erupted after ISIL gunmen fired into a crowd of civilians in the Aleppo village of KafrTakharim who were commemorating the death in ISIL custody of a prominent Syrian doctor and rebel commander, Hussein Suleiman.
Mr Suleiman's body was handed over by ISIL on Tuesday as part of a prisoner swap between rival rebel forces.
Video footage of his corpse showed signs of beating and one ear was cut off.