Insurgent factions in Syria have joined together to fight off an assault by a powerful jihadist group which launched an attack on rebels this week.
Several militias formed an alliance with key Islamist group Ahrar al-Sham amid some of the worst inter-factional clashes in recent times.
Jabhat Fateh al-Sham has accused the rebels of conspiring against it at peace talks in Kazakhstan this week.
The group was excluded from a ceasefire declared across Syria in December.
Jabhat Fateh al-Sham (JFS), known as al-Nusra Front until it broke off formal ties with al-Qaeda last July, is considered a terrorist group by the powers backing the ceasefire.
It is strongest in Idlib province, in north-western Syria, where it has previously fought alongside Ahrar al-Sham against pro-Syrian forces.
Idlib is one of the insurgents' last major territorial strongholds in Syria, following the fall of eastern Aleppo in December.
JFS has been involved in clashes with rebels in Idlib and neighbouring west Aleppo since Tuesday.
Ahrar al-Sham, which has rejected calls by outside powers to dissociate itself from JFS, blamed the jihadists for starting the fighting.
JFS accused rebel factions of launching "unprovoked" attacks on its bases in Idlib, killing six of its fighters, the Associated Press news agency reported.
As the fighting escalated on Wednesday, JFS militants stormed a prison in Idlib where members were being held by other rebel groups, according to the UK-based monitoring group, the Syrian Observatory for Human Rights (SOHR).
On Thursday, Ahrar al-Sham said JFS had rejected its attempts to mediate. The Islamists warned JFS that any attack on its members would be considered a "declaration of war", according to Reuters news agency.