The FSA militants have been laying siege to ISIL's headquarters in the city of Raqqa since last night, the so-called Syrian Observatory for Human Rights on Monday.

ISIL terrorists have captured the provincial capital city of Raqqa and it is said that they are holding hundreds of prisoners in their now besieged headquarters in the heart of Raqqa.

Among ISIL's abductees are scores of rival militants, activists and journalists, including Westerners.

Monday's offensive in Raqqa came three days after a new militant alliance called the Army of the Mujahideen declared war on ISIL.

On Sunday the rebel infighting spread to the central province of Hama, as well as Raqqa, and the Observatory says scores of militants have been killed on both sides.

The Syrian Observatory for Human Rights said "the main group laying siege to ISIL's headquarters in Raqqa is al-Nusra Front," which like ISIL is affiliated to al-Qaeda.

ISIL and al-Nusra have fought each other in recent months, after ISIL announced it was al-Qaeda's representative in Syria. Al-Nusra had been operating in Syria for longer, and refused to work under ISIL's command.

Al-Qaeda chief Ayman al-Zawahiri then ordered ISIL's Abu Bakr al-Baghdadi to work with al-Nusra -- and he refused. The two groups have since had, at best, tense relations, and at worst they have engaged in open fighting.

RA/NJF