Protesters have held daily demonstrations against jihadists in the northern Syrian city of Raqa, demanding the release of "hundreds" of missing people including an Italian Jesuit priest, a watchdog said on Monday.

News of the protests comes two weeks after Father Paolo Dall'Oglio went missing when he went to meet commanders of the jihadist Islamic State of Iraq and the Levant (ISIS) in late July to ask for the release of activists kidnapped by the group.

"Demonstrations have been held daily for two weeks demanding the release of hundreds of civilians" kidnapped by ISIS, including Dall'Oglio, said the Syrian Observatory for Human Rights.

The Britain-based watchdog described Dall'Oglio as a "messenger of peace" and "a friend of the Syrian opposition" to President Bashar al-Assad's regime.

Known for his charisma and good relations with peaceful anti-Assad activists, he has lobbied several Arab and Western officials to support the democratic opposition for fear of the rise of radical Islamism.

Raqa is the only provincial capital to have fallen out of regime hands since the start of Syria's protest movement in March 2011.

What began as a popular uprising has since morphed into a bloody insurgency after the regime unleashed a brutal crackdown against dissent.

The Observatory also reported on Monday clashes in Raqa five days ago that pitted ISIS against a local rebel group.

The fighting broke out after jihadists "attacked the headquarters of the Ahfad al-Rasul brigade in the Mahatta neighbourhood of Raqa", said the monitoring group.

In protests the day after, residents called jihadists "to leave" their area, the Observatory added.

"Syria is free! The (Islamic) State (of Iraq and the Levant) must get out!" protesters chanted.

In the early days of the uprising, when opponents of the regime were desperate for assistance from any quarter, jihadist fighters were welcomed.

But a spate of abuses is fuelling a backlash against the groups, particularly in northern Syria, swathes of which are out of army control.

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