Damascus: Syrian rebels have agreed a ceasefire with Al Qaida loyalists after bitter fighting for a key border town, a monitoring group said on Friday, as the opposition condemned the jihadist assault.

The National Coalition accused Al Qaid front group the Islamic State of Iraq and the Levant (ISIS) of violating the principles of the revolution by turning its guns on fighters of the mainstream Free Syrian Army (FSA) in Azaz.

ISIS seized the town on the border with Turkey in hours-long fighting on Wednesday, in the latest in a growing spate of clashes between jihadists and mainstream rebel units.

The Northern Storm brigade, which is loyal to the FSA and was based in Azaz, agreed to the truce with ISIS under which both sides pledged to observe a ceasefire, the Syrian Observatory for Human Rights said.

The deal was brokered by Liwa Al Tawhid, a powerful rebel brigade loyal to the FSA, which sent fighters to the town on Thursday who have deployed between the two sides, the Observatory said.

The rival groups also undertook to free detainees captured in Wednesday’s fighting and to immediately return any goods looted from the other side.

They agreed that any future problems that might emerge be dealt with by an arbitration committee, the Britain-based watchdog added.

Azaz has symbolic as well as strategic value as it was one of the first towns to be captured from government troops, in July 2012, by FSA fighters, who set up their own administration.

Tensions have spiralled between some mainstream rebel groups and ISIS in recent months, especially in northern Syria, where the opposition controls vast swathes of territory.

Several local groups resent ISIS’s growing territorial control, its steady supply of arms, as well as its brutality, which opponents often compare to that of the regime.

ISIS, on the other hand, has accused some rebels affiliated with the FSA’s Supreme Military Command of collaborating with the West and of being “heretics”.

The opposition National Coalition issued a rare condemnation of ISIS on Friday, accusing the group of violating the principles of the revolution by turning its guns on FSA fighters.