The UN's humanitarian chief Lise Grande said in an interview with The Associated Press that the global body expects the final battle for the Old City to start "within days". She said conditions there "are desperate" and the UN expects almost all civilians to try to escape.
US-backed Iraqi forces have been battling fighters from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) in Mosul since October. ISIL now control only a handful of neighborhoods in and around the Old City.
Grande said there have been more than 7,000 gunshot wounds of people trying to leave districts still controlled by ISIL.
"The reason we know that they're being shot at by snipers and not crossfire is because they're being shot in the back," she said.
Around 860,000 people have already fled Mosul, which was "beyond the worst case" scenario of 750,000 that the UN had planned for, said Grande.
The UN and Iraqi authorities have coordinated to build new camps and find spaces in existing camps for families returning to already recaptured eastern Mosul.
Grande said the Iraqi forces have tried to keep the daily flight of people below the saturation point of 20,000 when they attack a neighborhood.
"We're averaging between 8,000 and 15,000 a day," she said.
'It's going to get much worse'
Al Jazeera's Bernard Smith, reporting from the city of Erbil, east of Mosul, said that trapped civilians were reportedly without food or running out of water in the Old City, and that - while Iraqi forces have dropped leaflets urging residents to flee before the final battle for control of the area - ISIL are shooting people that try to leave and are even welding doors shut.
"All indications are that it's going to get much much worse for the civilians trapped in the city before it's going to get any better," said Smith.