The UN humanitarian chief in Iraq says the United Nations believes between 120,000 and 150,000 civilians are trapped in Mosul's Old City, where Islamic State extremists want to keep them as human shields -- and shoot at anyone trying to flee.

Lise Grande said in an interview Friday with The Associated Press that the UN expects the battle for the Old City to start "within days." She said conditions there "are desperate" and the UN expects almost everyone to try to get out.

She said 860,000 people have already fled Mosul -- a number "beyond the worst case" the UN expected.

U.S.-backed Iraqi forces have been battling IS extremists in Mosul since October. The militants now control only a handful of neighbourhoods in and around the Old City.