Tens of thousands of people have fled the Iraqi cities of Tikrit and Samarra after they came under attack from Islamist militants, the International Organisation for Migration said Friday, as it warned it is bracing for a drawn-out crisis.

Fighters from the Islamic State of Iraq and Greater Syria (ISIS) have overrun a succession of major towns and cities over the last week and are closing in on Baghdad.

The IOM, which is based in Geneva, said its sources on the ground estimate 40,000 people have fled militant-held Tikrit and Samarra, which has come under a series of attacks and where militants were gathering Friday for a new attempt to take the city.

The latest figure adds to the half a million people the IOM estimate to have fled Iraq's second city, Mosul, after it was overrun Tuesday.

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