US Secretary of State John Kerry has vowed "intense and sustained support" for Iraq after meeting key politicians in the capital, Baghdad.
He said attacks by Sunni militants were a threat to Iraq's existence, and the next days and weeks would be critical.
The insurgents are expanding their control of towns in the north and west.
They are bearing down on a vital dam near Haditha, and have captured all of the border crossings to Syria and Jordan from government forces.
The key airport in the northern town of Tal Afar has also fallen, with some reports saying the town itself has been captured.
Some serious shit is going down there ... 'again' :( Can US aid will be able to save Iraq from ISIS OT ?