The jihadist Islamic State of Iraq and the Levant has retaken the Syrian city of Raqa after fierce fighting for the northern provincial capital, a monitoring group said on Tuesday.
"ISIL took full control of the city of Raqa after days of clashes," the Syrian Observatory for Human Rights said.
The jihadist group has been battling a coalition of Islamist and moderate rebels in opposition-held areas across northern Syria.
Raqa is the only provincial capital the rebels have managed to fully prise from government control and subsequently became an ISIL stronghold.
A brigade loyal to the Al-Nusra Front, which is also Al-Qaeda-linked, has been battling ISIL fighters in Raqa, even though the group has largely stayed out of the fighting within rebel ranks elsewhere in Syria.
Al-Nusra is recognised by the Al-Qaeda leadership as the jihadist network's franchise-holder in Syria. It recognises ISIL's operational command only in neighbouring Iraq.
ISIL has been accused of committing horrific abuses in Syria as part of its quest for hegemony in opposition areas.
It is holding hundreds of rival rebels, activists and journalists captive, among them Westerners.
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The World Health Organisation and UN Children's Fund UNICEF said on Monday that the fighting in Raqa province had interrupted a polio vaccination campaign launched after 17 cases of the disease were reported.
The outbreak raised fears of a major setback to the international effort to eradicate a virus that was long one of the world's most dreaded childhood diseases, causing death or paralysis to hundreds of thousands.
Elsewhere in Syria, the Observatory said eight rebels were killed overnight when ISIL fighters detonated a car bomb in Idlib province in the northwest.
The group said the car bomb targeted a rebel checkpoint near Ram Hamdan, northeast of Idlib city.
"Eight fighters from Islamist and other rebel brigades died after a huge bombing carried out by ISIL fighters," the Observatory said.
Clashes between ISIL and coalitions of rival Islamist and moderate rebels erupted nearly two weeks ago.
Among those battling the jihadists is the Islamic Front which issued them an ultimatum late Monday.
"We are setting ISIL a deadline of no more than 24 hours to stop its abuses, free the prisoners and hand over its weapons," the front said.
In the central city of Homs, shelling killed 18 people as fighting continued between rebel forces and those of the regime of President Bashar al-Assad, the Observatory said.
More than 130,000 people have been killed in Syria since conflict erupted in March 2011.