Syrian Islamist rebels from the Islamic Front have captured a key Al Qaeda base in the northern city of Aleppo.

According to the Syrian Observatory for Human Rights, fighters found the bodies of several men who had apparently been executed, along with dozens of prisoners kept at a hospital in the Qadi Askar district. The hospital was being used as the headquarters for the Islamic State of Iraq and the Levant (ISIS), a branch of Al Qaeda.

The Observatory said that it was “unclear” what had happened to the apparently hundreds of ISIS fighters who had been holed-up inside the former children’s hospital.

ISIS – along with the Nusra Front – is one of two Al Qaeda-linked groups fighting in Syria against the regime of Bashar al-Assad. But despite sharing a common enemy tension between ISIS in particular and other rebel groups – including non-Al Qaeda-aligned Islamist brigades – have been steadily rising.

Other rebel groups accuse ISIS, which is made up largely of foreign jihadist fighters, of committing numerous crimes against Syrian civilians as part of a campaign to impose their hardline interpretation of Sharia (Islamic law) in the country, as well as working against the interests of the Syrian revolution in general.

ISIS has regularly clashed with rival rebel groups, including the recent summary execution of members of the Western-backed Free Syrian Army .

In recent days, several groups of Syrian rebels united for the purpose of launching a new “revolution” against ISIS, which they accuse of working with the Assad regime to undermine other rebel groups. Both the more secular Free Syrian Army and the Islamist Islamic Front have joined the fight against Al Qaeda, forcing the terrorist network to retreat from some areas in an attempt to consolidate its forces against the onslaught.

In response, ISIS has promised to “crush” those rebel groups fighting against it, declaring them “infidels”.

A recent video featured on Arutz Sheva showed ISIS fighters exchanging insults over a walkie-talkie with members of the Free Syrian Army, illustrating the extent of the enmity between the two factions.