Rebel fighters have laid siege to al-Qaeda-linked jihadists in their northern stronghold of Raqa, managing to free 50 people they had detained.
Raqa has emerged as a new front in fighting among rebels battling to oust President Bashar al-Assad, with various groups joining forces against al-Qaeda affiliate the Islamic State of Iraq and the Levant (ISIL).
"The rebels have been laying siege to ISIL's headquarters in the city of Raqa since last night. They released 50 Syrian prisoners held by ISIL in another building," said Syrian Observatory for Human Rights on Monday.
Raqa is the only provincial capital to have fallen out of regime hands since the conflict erupted when regime opponents took up arms following a bloody crackdown by Assad's forces on democracy protests in March 2011.
But soon afterwards it fell into the grip of ISIL, which is said to be holding hundreds of prisoners in their now besieged headquarters in the heart of Raqa.
Among ISIL's abductees are scores of rival rebels, activists and journalists, including Westerners.
Monday's offensive in Raqa came three days after three powerful rebel alliances, including moderates and Islamists, launched what they called a second "revolution" against ISIS in the northern province of Aleppo and Idlib to its west.
On Sunday, the rebel infighting spread to the central province of Hama, as well as Raqa, and the Observatory 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 jihadists refuse to operate within the broader opposition dynamic.
The Syrian Observatory for Human Rights said "the main group laying siege to ISIL's headquarters in Raqa is Al-Nusra Front," which like ISIL is affiliated to Al-Qaeda but is seen as more moderate.
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.
