Thousands of people have fled the Mosul area and more are expected as Iraqi troops move on the city to liberate it from the so-called Islamic State (IS), the BBC reports.
Some 5,000 people have crossed the border into Syria in the last 10 days, the aid agency Save the Children says.
They have arrived at the al-Hol refugee camp where conditions are already reported to be filthy and overcrowded.
Up to 1.5 million civilians are thought to still be in Mosul, with up to 5,000 IS fighters.
The UN is preparing for what it fears could be the biggest man-made humanitarian crisis in recent times.