The children’s hospital used as headquarters by the group appeared to be undamaged from government airstrikes in pictures released yesterday, giving some support to the theory that the forces of president Bashar al-Assad left the Isis base alone for strategic purposes, despite raining fire on other rebel-held parts of the city.
Critics of Isis within the rebel movement have accused the extremists of giving Mr Assad a valuable propaganda boost with their beheadings of enemies, brutal enforcement of Sharia law alien to Syria, and a heavy bias towards foreign fighters such as Chechens, Uzbeks and Muslims from western Europe.
One rebel fighter freed from the Isis prison yesterday said he had been detained after providing security for Syrians protesting against the group. He said he had been held for two months and ten days with his hands bound behind his back.
Photos released by the rebels who liberated the hospital showed at least a dozen men, hands tied, executed by shots to the head in an outdoor area of the hospital complex.
A spokesman for Isis vowed to crush the rebel groups attacking it.
“Know that we have armies in Iraq and Syria… we warn you,” said the spokesman known as Abu Mohammed al-Adnani in an audio clip posted on militant websites. “We will crush them and kill the conspiracy in its cradle.”
The statement from the Isis spokesman was issued late on Tuesday, hours after the head of the local al-Qaeda branch in Syria, the Nusra Front, called for a truce to halt the fighting.
While both the Nusra Front and Isis have roots in the global al-Qaeda network, the Nusra Front has focused its goals on toppling Mr Assad instead of creating an Islamic state, which has been the main aim of Isis.
A chaotic mix of rebel groups in Syria, including Islamic factions, turned their guns on fighters of the “Islamic State” last Friday. The clashes have since become the most serious rebel infighting since the uprising against Mr Assad began in March 2011.
The rebel-on-rebel fighting began after tensions, which had simmered for months, boiled over, following reports that al-Qaeda fighters had tortured and killed a popular doctor.
The fighting since has spread from the northern province of Aleppo to nearby Idlib and to the province of Raqqa, a bastion of Isis.
At least 300 people have been killed in the infighting in the past five days, Rami Abdurrahman, the head of the Britain-based Syrian Observatory for Human Rights said yesterday.
The capture of the hospital was a boost for the rebels, who on Tuesday saw 20 of their fighters killed in an al-Qaeda suicide car bombing in the northern city of Darkoush, according to activists.