Islamic State militants took full control of the western Iraqi city of Ramadi on Sunday in the biggest defeat for the Baghdad government since last summer.
In a statement, the group said it had seized tanks and killed "dozens of apostates", its description for members of the Iraqi security forces.
Ramadi is the capital of Iraq's western Anbar province, which is dominated by Sunni Muslims. Prime Minister Haider al-Abadi signed off on the deployment of Shi'ite militias to attempt to seize back the area, a move he previously resisted for fear of provoking a sectarian backlash.
Earlier, security sources said government forces evacuated a key military base after it came under attack by the insurgents, who had already taken one of the last districts still holding out.
It was the biggest victory for Islamic State in Iraq since security forces and Shi'ite paramilitary groups began pushing the militants back last year, aided by air strikes from a U.S.-led coalition.