Iraq's refugee population has increased by almost 800,000 this year as the government struggles against rebels and Islamic militants, the United Nations says.

The UN's refugee agency said 300,000 people fled this week alone in Irbil and Duhok as Islamic militants seized control of large areas in northern Iraq.

UNHCR spokesman Adrian Edwards said many of the refugees are arriving with little more than the clothes they wear and have no money and nowhere to go.

Last week, the agency reported that nearly 480,000 people had been driven from their homes since January by fighting between government forces and rebels in western Iraq's Anbar province.