The UN says hundreds of thousands of people have fled the Iraqi city of Mosul, since it was overrun by Islamist militants.

Insurgents from the Islamic State of Iraq and the Levant seized the provincial governor's headquarters on Tuesday, and rampaged through police stations, military bases and the airport. About 10,000 Mosul residents sought refuge in the Kurdish province of Irbil.

"We protected and saved thousands of people who came from Mosul at the checkpoint and we have decided to build a camp for those people. We will support them with water and food," Irbil governor Nawzad Hadi said.

Kurdish authorities tightened security at checkpoints in Irbil, to make sure Islamic militants could not enter the autonomous region. Security officials in neighboring Dahuk also said 5,000 displaced people had fled to the province.

A UN spokesperson says an estimated 500,000 people have fled Mosul. The United Nations Assistance Mission for Iraq has expressed willingness to offer support.