More than 50 hostages freed as rebel groups in Syria make sweeping advances against Islamic State of Iraq in Syria

Syrian rebels have ousted a hardline al-Qaida group from the provincial capital of Raqqa, freeing more than 50 hostages in a fourth day of clashes across the north of the country.

The fight against the group, the Islamic State of Iraq in Syria (Isis), comes as members of the same group remain in control of much of Ramadi and Falluja, despite similar attempts to oust them by the Iraqi military.

Rebel groups in Syria have made sweeping advances against Isis since first taking on the powerful militia on Friday. Since then, many of its members have withdrawn from most of the Turkish border areas it had held for at least six months.

Others have left the group to join another al-Qaida organisation, Jabhat al-Nusra, or more mainstream opposition groups, including the remnants of the Free Syria Army and a powerful new alignment of Islamic units.

Raqaa is the only provincial capital to have fallen out of the hands of the Syrian regime. Held first by the Free Syria Army, by June last year had become a stronghold for Isis, which then imposed a ruthless interpretation of sharia law on what remained of the town's population.

Scores of captives, among them journalists and aid workers, had been detained by Isis in government buildings. Those freed on Monday appeared to all be Syrians. It is understood that the western captives had earlier been moved to another location.

In Iraq, the prime minister, Nouri al-Maliki, on Monday urged tribal leaders to unite with the country's military, which claims to have surrounded the two Sunni cities west of Baghdad and to be preparing to attack.

The US has provided hellfire missiles to Iraq, which are fired from Cessna planes flown by Iraqi pilots. It has also provided surveillance drones. It is not clear whether US officials are guiding the drones, or whether any of them are able to fire missiles.

Tribal figures have significant influence in both cities and in the surrounding Anbar province. Some have aligned with the Isis militants, who entered urban areas after the Iraqi army withdrew a week ago. Others have allied with Maliki's military, which is perceived by many in Anbar as an army of Shia soldiers and officers.

Iran said it had also offered assistance to Maliki in launching an attack. Accepting such an offer would likely prove difficult for the Iraqi premier, who has been trying to reassure Anbar leaders that he is fighting extremists, and not the Sunni sect.

Reinforcements from Syria are continuing to move towards Falluja and Ramadi, Iraqi officials say, making the going easier for the Syrian opposition in its fight against Isis. Rebel groups claim to have secured much of eastern Aleppo, which has remained out of the hands of the Syrian regime for 18 months, during which some districts had been taken over by Isis.