"Oh Sunni people, you were forced to take up the weapon... do not lay the weapon down, because if you put it down this time, the (Shiites) will enslave you and you will not rise again," spokesman Abu Mohammed al-Adnani of the Islamic State of Iraq and the Levant said.
ISIL militants and anti-government tribesmen have battled Iraqi security forces and their tribal allies in Anbar province for more than a week, seizing control of part of provincial capital Ramadi and all of Fallujah.
Soldiers are deployed near Fallujah, which remains fully outside government control, but have yet to begin an assault to retake it.
It is the worst violence to hit Anbar province in years, and the first time militants have exercised such open control in major cities since the height of the insurgency that followed the 2003 US-led invasion.