An Al-Qaeda-linked group urged anti-government fighters locked in a deadly standoff with Iraqi forces to "creep towards Baghdad", in an audio message posted online Sunday.

The remarks, purportedly from the head of the Islamic State of Iraq and the Levant (ISIL), come amid a weeks-long security crisis in Iraq's western province of Anbar where the authorities have lost control of an entire city and parts of another.

"This is your chance, do not waste it," said the voice in the message, apparently that of Abu Bakr al-Baghdadi, addressing Sunni Arabs.

"Sons of the (Islamic) state in Iraq, you are stronger than a mountain," he continued.

"Be the spearhead in fighting the Safavids, stay on the frontline and creep towards Baghdad and to the south, and keep the Rawafadh busy on their doorstep," he said, using pejorative terms for Iraq's Shiite majority.

ISIL has been active in the fighting in Anbar province, but so have other militant groups and anti-government tribesmen.

The Iraqi security forces have also been involved, and have recruited their own tribal allies in a bid to retake the parts of Ramadi and all of Fallujah that have fallen out of government control.

The fighting first erupted in the Ramadi area on December 30, when security forces cleared a year-old Sunni Arab protest camp.

It spread to Fallujah, and militants moved in and seized the city and parts of Ramadi after security forces withdrew.

It marks the first time anti-government fighters have exercised such open control in major cities since the height of the insurgency that followed the US-led invasion of 2003.