The U.N. refugee agency says that more than 100,000 people have been displaced as Iraqi forces clear territory ahead of the critical battle for the Islamic State-held city of Mosul.

Iraqi leaders have repeatedly promised that Mosul — in IS hands for more than two years — will be retaken from the militant group this year, though U.S. officials have said that timeline is unrealistic.

Now, aid groups worry that a million more could be displaced from in and around Mosul as the operation moves forward.

The UNHCR said this week that about 43,000 people have been displaced from the Mosul area since March and 66,600 people from the nearby Shirgat area since June.

Available camps for the displaced are already overflowing from the influx, leaving thousands without shelter.