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Raqqa is the only provincial capital to have fallen out of regime hands since the conflict erupted after a bloody crackdown by Assad's forces on democracy protests in March 2011.
But soon afterward, it fell into the grip of the ISIL, which is said to be holding hundreds of prisoners in their now besieged headquarters in the heart of Raqqa.
One of those captives, Turkish photographer Ben Aygun from the newspaper Milliyet, who was taken hostage in mid-December, was released Monday.
A spokesman for the Islamic Front told Al Jazeera his group had freed Aygun and transported him to Turkey via the Bab al-Hawa border crossing.
"Every night, I had the same dream that I was being freed. I cannot believe that I am free now. It feels like a dream," Milliyet quoted Aygun as saying.
Monday's offensive in Raqqa came three days after three powerful rebel alliances launched what they called a second revolution, against the ISIL in the northern provinces of Aleppo and Idlib.
On Sunday the rebel infighting spread to the central province of Hama, as well as Raqqa, and the Syrian Observatory for Human Rights says scores of insurgents have been killed on both sides.
A key complaint against ISIL among rebels — including the massive Islamic Front, the Syrian Revolutionaries Front and nascent Mujahedeen Army — is that its extremist fighters refuse to operate within the broader opposition dynamic.
The ISIL, as its name suggests, seeks to lay the foundation for the restoration of an Islamic caliphate in Iraq and Syria. Removing Assad from power is the first step the fighters want to take, and their aims initially aligned with moderate Islamist and civil-state-minded rebel factions.
The Syrian Observatory for Human Rights said "the main group laying siege to ISIL's headquarters in Raqqa is Al-Nusra Front," which, like the ISIL, is affiliated with Al-Qaeda but is composed primarily of homegrown Syrian fighters and has been more cooperative with other rebel factions.
The ISIL and Al-Nusra have fought each other in recent months, after the ISIL announced it was Al-Qaeda's representative in Syria. Al-Nusra has been operating in Syria longer and refused to work under the ISIL's command.
The 33-month conflict in Syria is estimated to have killed more than 130,000 people and forced millions more to flee their homes as refugees or internally displaced people.
Al Jazeera with wire services