A number of Sunni tribal sheikhs and tribes in Iraq's Anbar province have pledged allegiance to the Islamic State in Iraq and the Levant (ISIL), a defection that comes as a major blow to the Iraqi government as it struggles to roll the Sunni insurgents back.

The sheikhs and tribal leaders made the pledge in a statement read out by influential Sheikh Ahmed Dara al-Jumaili, after a meeting in Fallujah on Wednesday. It was not yet clear if the tribes had been forced to pledge allegiance by ISIL fighters, who control Fallujah and most of Anbar province, and have been known to massacre even fellow Sunnis who stand against them.

The sheikhs’ statement said the only way peace would come to Anbar province would be if the tribes joined ISIL. They said they were joining ISIL’s self-declared “caliphate” in order to “fight the infidels, apostates and Shias,” using a derogatory term to refer to them.

If the statement was given freely, the move would be very worrying for the Iraqi government. The inclusion of the al-Jumaili tribe in Wednesday’s pledge was of particular concern for Iraqi authorities, given the tribe's influence in the contested Anbar province.

But the pledge comes after a number of Sunni leaders in Anbar publicly criticized the involvement of hardline Shia militias in the fight to retake areas of the province from ISIL, including the provincial capital city of Ramadi, which fell last month.

The underpowered Iraqi army, which wilted before the ISIL onslaught last June, has been forced to rely heavily on these Tehran-backed Shia militias, which have a history of sectarian reprisals against Iraq’s Sunnis. Many Shias — who form the majority in Iraq and control most of the government in Baghdad — blame the Sunnis for allowing ISIL to take over large swaths of Sunni-majority land in northern and western Iraq.

Despite their misgivings, a number of Sunni tribes have nonetheless joined U.S.-backed government forces and the Shia militias in fighting ISIL. But many tribal leaders say the government is not providing them enough support, citing numerous instances in which ISIL has stormed Sunni villages and slaughtered men, women and children.