Clashes between Syrian rebels and their rivals from an al-Qaeda-linked faction have spread from the country's opposition-held areas in the north to a key eastern city, activists have said.

Monday's rebel-on-rebel fighting in the eastern city of Raqqa, a long-time bastion of an al-Qaeda-linked group known as the Islamic State in Iraq and the Levant (ISIL), reflects a widening war within a war in Syria, as the Free Syrian Army tries to quell more hardline elements of the opposition.

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The recent infighting has been the most serious since armed groups initially rose to try overthrow the rule of Syrian President Bashar al-Assad.

The clashes erupted in the northern provinces of Aleppo and Idlib on Friday after residents there accused ISIL fighters of killing a popular doctor.

An activist group, the British-based Syrian Observatory for Human Rights, has estimated that at least 100 fighters on both sides have been killed since Friday.

ISIL is dominated by foreign fighters who initially fanned into Syria from neighbouring Iraq in March and set up camp in areas rebels had wrested from Assad-loyal forces, imposing a deeply conservative interpretation of Islamic law in the process.

Popular resentment towards the group from civilians has been building as the months progress and ISIL fighters began imprisoning citizens for perceived transgressions against Islam.

The rebels, meanwhile, have accused ISIL of overtaking their areas, seizing their weapons and detaining their fighters.

Rebel alliance

Raqqa 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 protests in March 2011.

But it soon fell into the grip of ISIL, which is said to be holding hundreds of prisoners in their now besieged Raqqa headquarters, among them rival rebels, activists and journalists.

Monday's offensive in Raqqa came after three powerful rebel alliances on Friday launched what they called a second "revolution" against ISIL in the northern province of Aleppo and Idlib to its west.

"The foreigners are being held in other buildings, outside Raqqa city," said the Observatory.

Another activist group, the Local Coordinating Committees, also reported the Raqqa clashes, saying they were focused around a city post office.

The Observatory said rebels had surrounded ISIL's chief compound in Raqqa and had liberated at least 50 detainees from a nearby prison.

Source: Al Jazeera and agencies