Around half a million people have fled their homes since Iraqi forces launched an operation to retake Mosul from Daesh exactly six months ago, said the UN on Monday.
Iraqi forces recaptured the east side of Mosul in January as the operation in the country's second largest city started on October 17, 2016.
But an offensive that was launched the following month on the part of Mosul that lies west of the Tigris river has increased the numbers of displaced people.
"The sheer volume of civilians still fleeing Mosul city is staggering," Lise Grande, the UN's humanitarian coordinator in Iraq, said in a statement.
"Our worst case scenario when the fighting started was that up to one million civilians may flee Mosul. Already, more than 493,000 people have left, leaving almost everything behind," she said.
The UN has estimated that another half million civilians were still in Daesh-controlled areas of west Mosul.
'Pushed to the limit'
Around two-thirds of the overall number of displaced people have fled their homes in west Mosul over the past two months alone.
More than half of those who have fled Mosul and its surroundings are children and the UN is concerned that those still in Daesh-controlled areas would be more exposed than ever before.
Grande said the fighting there was tougher than on the east bank, adding "there are more trauma injuries, homes are being destroyed, food stocks are dwindling quickly and families are at serious risk because there isn't enough drinking water."
"Mosul has pushed us to our operational limits," said Grande.
UN chief Antonio Guterres said on March 31 during a visit to a displacement camp near Mosul that the aid effort was woefully underfunded, calling for greater international solidarity.
"We have seen children with signs of psychological distress while others have been injured in the fighting, or used as human shields," the UN Children's Fund said.
In east Mosul, many of those displaced in the early stages of the operation have returned while the city's key infrastructure destroyed.
The UN has been expanding the capacity of some of the camps scattered around Mosul but the aid community could yet have to deal with an unprecedented exodus if and when the remaining civilians flee the city.