Iraq's military and some tribal leaders have struck a deal to drive out al-Qaeda-linked fighters in Anbar amid continued clashes and shelling in the predominantly Sunni Arab western province.

Pro-government Sunni tribes have asked the Iraqi military to withdraw from Anbar, and together with local police tribal fighters have begun to retake checkpoints and other vital government installations.

Fallujah and parts of the Anbar provincial capital, Ramadi, farther west have been outside government control for days - the first time al-Qaeda-linked fighters have exercised such open control in major Iraqi cities since the height of the insurgency that followed the US-led invasion of 2003.

The violence in Anbar, which began on December 30 with the removal of a Sunni Arab anti-government protest camp, has killed more than 250 people - the worst unrest to hit the province in years.

Al Jazeera's Imran Khan reported quoting local tribal sources on Wednesday that an interim deal had been reached with a view to defeating the Islamic State in Iraq and the Levant (ISIL).

"The arrangement calls for the Iraqi army to withdraw from the outskirts of Fallujah and Ramadi and to move to the outskirts of Anbar," he said.

"Pro-government tribes in conjunction with local police have begun to retake police checkposts on the edge of Fallujah. The police force is much less of an issue for the Sunni Arab tribes as police officers have been recruited from the tribes themselves.

"Many Sunni leaders are suspicious of the Iraqi army's presence as they see it a tool of the Maliki government. Tribal leaders have called on ISIL fighters to leave Fallujah to avert a full-scale assault on the city by pro-government Sunni tribes."

The developments come amid an announcement by the Iraqi Red Crescent that more than 13,000 families have fled Fallujah in the past few days.

"Most of them are now living in schools, public buildings or with relatives," Mohammed al-Khuzaie, a Red Crescent official, said in a statement on Wednesday.

Khuzaie said Red Crescent had provided humanitarian assistance to more than 8,000 families in the past three days across Anbar.

'Critical situation'

Separately, the top UN official in Iraq said that a "critical humanitarian situation" was developing in Anbar.

Nickolay Mladenov said on Wednesday the humanitarian situation in the province was likely to worsen as military operations to rout the fighters continue around Fallujah and Ramadi.

He said stocks of food, water and medicine in Fallujah were beginning to run out, and that more than 5,000 families had fled to neighboruing provinces to escape the fighting.

Life inside Fallujah appeared to be returning to some semblance of normalcy on Wednesday, though the situation remained tense.

A call went out over mosque loudspeakers late on Tuesday urging fleeing families to come back and to fighters to leave the city.

Some markets reopened and some families returned to their homes, while civilian cars and goods vehicles were seen driving through the city and traffic policemen were on the streets.

For his part, Nouri al-Maliki, Iraq's prime minister, has urged all those who joined al-Qaeda and now face a siege by his troops to give up their struggle, hinting of a possible pardon if the fighters give up the fight.

Speaking in his weekly televised address on Wednesday, Maliki also pledged to continue the "sacred war" against ISIL and finish the push to retake Fallujah and Ramadi.

"I call on those who were lured to be part of the terrorism machine led by al-Qaeda to return to reason," he said.

In return, he promised that his government will "open a new page to settle their cases so that they won't be fuel for the war that is led by al-Qaeda".

Maliki's comments came a day after Iraqi security officials announced the killing of 25 al-Qaeda-linked fighters in an air raid in the outskirts of Ramadi.

The Defence Ministry did not give more details about how the death toll was confirmed but cited intelligence reports. It was not possible to independently verify the military's claim.

The attack came after clashes erupted about 20km west of Fallujah, following the capture of an army officer and four soldiers in the area on Monday, Dhari al-Rishawi, a provincial spokesman, said.