Militants were laying siege Monday to Al-Qaeda-linked extremists in their northern stronghold of Raqa, managing to free 50 people they had detained, said the opposing UK-based Syrian Observatory for Human Rights.

Militants were laying siege Monday to Al-Qaeda-linked extremists in their northern stronghold of Raqa, managing to free 50 people they had detained, said the opposing UK-based Syrian Observatory for Human Rights.

Raqa emerged as a new front Sunday in fighting among militants battling the Syrian government, with various groups joining forces against Al-Qaeda affiliate the Islamic State of Iraq and the Levant (ISIL).

"The militants have been laying siege to ISIL's headquarters in the city of Raqa since last night. They released 50 Syrian prisoners held by ISIL in another building," said the Observatory.

On Sunday the rebel infighting spread to the central province of Hama, as well as Raqa, and the Observatory says scores of militants have been killed on both sides.

The Syrian Observatory for Human Rights said "the main group laying siege to ISIL's headquarters in Raqa is Al-Nusra Front."

ISIL and Al-Nusra have fought each other in recent months, after ISIL announced it was Al-Qaeda's representative in Syria. Al-Nusra had been operating in Syria for longer, and refused to work under ISIL's command.

Al-Qaeda chief Ayman al-Zawahiri then ordered ISIL's Abu Bakr al-Baghdadi to work with Al-Nusra -- and he refused. The two groups have since had, at best, tense relations, and at worst they have engaged in open fighting.