The sheikhs and tribal leaders made the pledge on Wednesday in Fallujah in a statement read out by Ahmed Dara al-Jumaili, an influential sheikh, after a meeting.
Al Jazeera's Imran Khan, reporting from Baghdad, said it was not yet clear if the tribes had been forced to pledge allegiance by ISIL fighters, who control Fallujah and most of Anbar.
"The statement they issued was very strong - it condemned the government.
"It said the only way that peace would come to Anbar province is if the tribes joined ISIL."
Our correspondent said the inclusion of the al-Jumaili tribe in the pledge was of particular concern for Iraqi authorities, given its influence in Anbar.
"The al-Jumailis command a number of fighters and they have a large amount of influence over other tribes [in Anbar]," he said.
Matthew Henman, an expert on terrorism and insurgencies, told Al Jazeera that ISIL may have threatened the Sunni tribes into backing the group.
"But it could also be symptomatic of a wider break in trust of the Iraqi government and a feeling of a lack of confidence in Baghdad's ability to recapture territory from ISIL, leaving them to the mercy of ISIL and little option but to side with them," he added.
The pledge comes after a number of Sunni leaders in Anbar publicly criticised the involvement of Shia units in the fight to retake areas of the province from ISIL, including the provincial capital Ramadi, which fell last month.
While a number of Sunni tribes have joined with government forces and Shia units, Al Jazeera's Khan reported that a number of tribal leaders have asked for government support to fight ISIL.
"They said 'If you arm us, if you allow us to fight as Sunnis, we will be able to get rid of ISIL quite quickly'," he said.
"The fact that a number of these tribes have come together ... and pledged allegiance to ISIL shows the level of anger the Sunni tribes feel towards the government in Baghdad."
