The fighting in the Islamic State-held Old City of Mosul, where hundreds of thousands of Iraqi civilians are trapped, could turn into the worst humanitarian “catastrophe” in the war against the militants, the United Nations warned on Tuesday.

“If there is a siege and hundreds of thousands of people dont have water and dont have food, they will be at enormous risk,” U.N. humanitarian coordinator for Iraq Lise Grande told Reuters in a telephone interview.

“We could be facing a humanitarian catastrophe, perhaps the worst in the entire conflict,” she added.

Mosul, the largest city in northern Iraq, was captured by the ultra-hardline Sunni Muslim fighters in mid 2014.

Iraqi government forces have taken back most of it in a U.S.-backed offensive launched in October, including the half that lies east of the Tigris river.

The militants are now surrounded in the northwestern quarter including the historic Old City, using booby traps, sniper and mortar fire against the assailants.