The United Nations says nearly half a million civilians have fled Mosul since U.S.-backed Iraqi forces launched a wide-scale military operation last October to retake the city from Daesh militants.

The U.N. Office for the Coordination of Humanitarian Affairs said Monday that 493,000 people have been displaced from the city. As many as 500,000 others are in Daesh-controlled parts of western Mosul, where fighting is still underway.

"The sheer volume of civilians still fleeing Mosul city is staggering," Lise Grande, the UN's humanitarian coordinator in Iraq, said in a statement.

More than half of those who have fled Mosul and its surroundings are children and the U.N. voiced concern that those still in Daesh-controlled areas would be more exposed than ever before. "We have seen children with signs of psychological distress while others have been injured in the fighting, or used as human shields," the U.N. Children's Fund said. The U.N. says food, water and medicine stocks are running low in the western half of the city, and that the fighting there is much heavier than it was in eastern Mosul, which the Iraqi government declared "fully liberated" in January.