Iraqi authorities and aid agencies are already struggling to cope with a surge in displacement since security forces opened a new front against the militants in Mosul earlier this month.
Iraqi forces have dislodged Daesh from all but about 12 square km (5 square miles) of the city and are seeking to claim victory before the holy month of Ramadan in less than two weeks.
The militants, however, still control the Old City, where they are expected to make their last stand in the densely populated, narrow streets that are impassable for armored vehicles.
"As military operations intensify and move closer to Mosul’s Old City area, we expect that up to 200,000 more people will flee,” Lise Grande, the UN's Humanitarian Coordinator for Iraq said in a statement, describing the figures as "alarming", Reuters reported.
“The numbers of people who are moving are now so large, it’s becoming more and more difficult to ensure civilians receive the assistance and protection they need."
Nearly 700,000 people have fled Mosul since the start of the campaign to retake the city last October, seeking refuge either with friends and relatives or in camps.