Syrian rebels have laid siege to jihadists from the Islamic State of Iraq and the Levant in their northern stronghold, hoping to crush the al-Qaeda affiliate accused of widespread abuses.

A broad coalition of moderates and Islamists opposed to President Bashar al-Assad is seeking to drive ISIL – which is accused of kidnapping, torturing and killing rival rebels and civilians – from its stronghold in the northern city of Raqa.

The new front in Syria’s increasingly complex civil war cracked open less than three weeks away from a planned peace conference, for which the United Nations has started sending out invitations, excluding Assad’s ally Iran.

The Syrian Observatory for Human Rights said rebels in Raqa managed to free 50 Syrian prisoners held by the Sunni extremists, who are believed to be holding hundreds of prisoners, including foreign journalists.

Raqa is the only provincial capital lost by the regime since the start of the conflict, which began in March 2011 with peaceful protests demanding democratic reform but escalated into a full-blown war when Assad’s forces launched a brutal crackdown on dissent.

The city later fell into the hands of ISIL, the latest incarnation of al-Qaeda’s Iraq affiliate, which joined the fight against Assad’s regime in late spring 2013.

The rebels initially welcomed the battle-hardened jihadists, but tensions mounted as ISIL was accused of imposing a reign of terror in areas where it operates, especially Raqa.

Three powerful rebel alliances on Friday launched what activists called a second “revolution”, and have advanced quickly, expelling ISIL from checkpoints and bases across Aleppo, Idlib and Hama provinces.