The advance by the jihadist Islamic State of Iraq and the Levant has forcibly displaced 850,000 people in one month and has raised to 1.6 million the total number of people who have evacuated their homes since January, the International Organization for Migration said Friday.
Most of those who have fled have found refuge in the autonomous Iraqi region of Kurdistan, the IOM said.
"Such overwhelming numbers point to a longer-term crisis - one that may find many more in need of critical lifesaving assistance," the IOM's emergency coordinator for Iraq, Brian Kelly, said in a statement.
Many of the people fleeing into the Kurdish region "have already been on the move for weeks and months," he said.
An overwhelming number of the refugees now living in churches, mosques, schools, parks and buildings under construction without running water or electricity, were originally from Mosul, Iraq's second city, and the town of Sinjar.
Iraq has been the scene of sectarian conflict since last June, when Sunni insurgents launched a powerful offensive in the northern half of the country.
Proof exists that these jihadists have been responsible for grave crimes and acts of extortion against the civilian population.
Their chief victims have been Shiite Muslims and religious minorities, such as Christians and Yazidis.