An extremist group embroiled in infighting with one of the largest Syrian rebel groups has pledged allegiance to al-Qaida-linked militants, a move that threatens to deepen divisions in the already-disparate insurgency.
The extremist group Jund al-Aqsa has been battling the powerful Ahrar al-Sham group in northern Syria for days, after the latter accused it of ambushing its leaders.
Others said the fighting was triggered by the ultra-conservative Ahrar al-Sham that attacked positions of the extremist Jund al-Aqsa group in several places.
The bloody infighting left a number of Ahrar al-Sham members dead and also stalled an offensive led by Jund al-Aqsa against government troops in the central Hama province.
Scores of Syrian rebel factions rallied behind Ahrar al-Sham, accusing Jund al-Aqsa of espousing extremist ideologies and seeking to dominate other rebel groups. In various statements, rebel factions accused Jund al-Aqsa of serving the extremist Islamic State militants, who are at odds with all Syrian insurgents, including the al-Qaida-linked Fatah al-Sham Front.
On Sunday, Fatah al-Sham Front announced Jund al-Aqsa would join its ranks "to stop the bloodshed".
The hand-written declaration was signed by Fatah al-Sham's leader Abu Mohammed al-Golani and his local commander of Jund al-Aqsa, Abu Diab al-Sarmini.
The declaration was expected to put an immediate end to the infighting because Jund al-Aqsa will now enjoy the protection of the powerful Fatah al-Sham Front.
But the Syrian Observatory for Human Rights reported intense fighting after the declaration between Jund al-aqsa and Ahrar al-Sham in the northern province of Idlib.
The declaration has also drawn criticism from other rebel groups, however, who see the move as divisive, providing cover for an extremist group.
Charles Lister, a researcher who has focused on the al-Qaida affiliate, said the move constituted a "huge risk" for the Front, risking alienating other rebel groups.
A spokesman for Ahrar al-Sham, Ahmed Karali, said his group was not consulted about the move and vowed it would continue its fight against Jund al-Aqsa.
Yasser Alyousef, a member of the Nour el-Din el-Zinki rebel group, said his group would hate to see Fatah al-Sham offer "refuge to extremists", warning that it may lead to another incarnation of the Islamic State group.
"All factions have united to fight against Jund al-Aqsa until it dissolves or is uprooted," Alyousef said. "Any attempt to dilute the issue... or offering superficial solutions is treason to the blood of martyrs."
The Fatah al-Sham Front has been working to improve its image and distance itself from al-Qaida. In July, its leader, al-Golani, publicly showed his face for the first time in a video in which he announced the group's name change and said it was cutting ties with the international terror organisation.