Al-Qaeda's general command has disavowed all links with the Islamic State in Iraq and the Levant (ISIL), according to a statement posted online.

The statement, published late on Sunday, reiterated a previous peremptory statement in which the group's chief Ayman al-Zawahiri ordered ISIL to disband and return to Iraq, and adding that Jabhat al-Nusra was al-Qaeda's official branch in Syria.

"Al-Qaeda announces it is not linked to the Islamic State of Iraq and the Levant, as it was not informed of its creation [and] did not accept it," read Sunday's statement, which criticised ISIL's mode of operations.

ISIL "is not a branch of al-Qaeda, has no links to it, and the [al-Qaeda] group is not responsible for its acts," it added.

"We affirm our disavowal from the sedition that is occurring in Syria between factions of jihadists, and from the blood that was shed by any party," it said.

Uprising undermined

In recent weeks, ISIL consolidated its grip on the northern city of Raqa, the only provincial capital to fall out of regime control since the outbreak of Syria's uprising in March 2011, imposing their strict version of Sharia law on residents.

ISIL also issued four statements ordering women to wear the niqab in public, forbidding the sale of cigarettes and narghile (water pipe) products, banning music and making attendance of Friday prayers compulsory.

It has also killed the leaders of two rival rebel groups on Saturday.

Small but powerful, ISIL has been caught up in battles with other armed groups often triggered by disputes over authority and territory, and has also clashed with secular rebels.

The internecine fighting - among the bloodiest in the three-year conflict - has undermined the uprising against President Bashar al-Assad and dismayed Western powers pushing for peace talks.

Rebel-on-rebel violence in Syria has killed at least 1,800 this year alone. The tensions erupted in early January into armed clashes between ISIL and other rebel groups.

Charles Lister, visiting fellow at Brookings Doha Centre, said the al-Qaeda statement "represents an attempt to re-assert some level of authority over the jihad in Syria" following a month of fighting and ISIL disobedience.

"This represents a strong and forthright move by [al Qaeda] and will undoubtedly serve to further consolidate Jabhat al-Nusra's role as al-Qaeda's official presence in Syria."