Fierce clashes erupted in northwestern Syria on Tuesday between extremist opposition fighters, clinging to their stronghold, and moderate rebel groups.

Free Syrian Army (FSA) groups, some of whom have representatives attending peace talks in Kazakhstan, accused Jabhat Fateh al-Sham of launching a surprise attack on their positions west of Aleppo.

Fateh al-Sham could not immediately be reached for comment. FSA officials said the attack targeted FSA groups in areas west of Aleppo and the adjacent province of Idlib, which is controlled by an array of rebel factions, including Fateh al-Sham.

One FSA commander said that in response “a comprehensive war” was now underway against Fateh al-Sham.

Long-standing tensions between Fateh al-Sham and more moderate rebels, a number of them backed by Turkey and other states that have opposed authoritarian Bashar al-Assad, have surfaced again since regime forces helped by Russian air power and Iranian-backed militias drove the rebels out of Aleppo last month.