In the north, the Kurds have been the main line of defence against the militants, but their fighters are stretched over a long front trying to fend them off.
The US has launched a number of military airstrikes in northern Iraq to hold off Islamic State forces advancing on Irbil, the capital of Iraq's autonomous Kurdish region.
US President Barack Obama ordered the intervention after militants used artillery to shell Kurdish forces defending Erbil, which is home to a US consulate and about three dozen US military trainers.
The US also airdropped aid to thousands of members of the Yazidi religious minority who fled to a mountaintop after the militants took control of Sinjar, a city far from the Kurdish seats of power in Irbil and Suliminiya.