A number of Sunni tribal sheikhs and tribes in Iraq's Anbar province have pledged allegiance to the Islamic State of Iraq and the Levant (ISIL) group.

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.

"If this is a willing move, then that is very worrying for the Iraqi government," he said.

"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 said.

The Anbar sheikhs' pledge comes after a number of Sunni leaders in the province publicly criticised the involvement of Shia units in the fight to retake areas from ISIL, including the provincial capital Ramadi, which fell last month.

Government's support

While a number of Sunni tribes have joined with government forces and Shia units, Al Jazeera's Khan said a number of tribal leaders had 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."

In another Anbar development, ISIL attacked a base north of Fallujah with two explosives-rigged vehicles driven by suicide bombers, and another south of the city with four more, including a bulldozer, an army colonel said on Thursday.

The attacks were foiled using Russian Kornet anti-tank missiles, the officer said.

Against this backdrop, Iraqi officials have also accused ISIL of cutting off water supplies to government-held areas in Anbar.

On Wednesday, fighters closed the locks on a dam on the Euphrates River near Ramadi, reducing the flow downstream and threatening irrigation systems and water treatment plants in nearby areas controlled by troops and tribes opposed to ISIL.

Taha Abdul-Ghani, an Anbar councilman, said the move will not only make the lives of people living in the affected areas more difficult but it could also pose a threat to the security forces fighting to recapture Ramadi. If water levels drop significantly, he said, the fighters could cross the Euphrates River on foot.

Thousands of people in government-held towns of Khalidiya and Habaniya are already suffering from shortages of drinking water because purification plants along the Euphrates have all but shut down because of already low water levels on account of the summer weather.

Also on Thursday, an army officer said security forces used anti-tank missiles to repel suicide bombers driving explosives-rigged vehicles who attacked two military bases west of Baghdad.