UNITED NATIONS: The United Nations believes up to 150,000 civilians are trapped in Mosul's Old City where Islamic State extremists want to keep them as human shields — and are shooting people trying to flee, the UN humanitarian chief in Iraq said Friday.

Since October, US-backed Iraqi forces have been battling IS militants in Mosul, Iraq's second-largest city. The extremists now control only a handful of neighborhoods in and around the Old City.

Lise Grande, the UN deputy special representative and humanitarian coordinator in Iraq, said in an interview with The Associated Press that the United Nations expects the battle for the Old City to start "within days."

She said that "conditions in the Old City are desperate," with very little food and no clean water, and the UN expects almost everyone to try to get out — "probably 120,000 to 150,000 civilians."

Already, 860,000 people have fled Mosul, which was "beyond the worst case" scenario of 750,000 that the UN had planned for, Grande said.

But she said extensive planning and coordination with Iraq's military had allowed the humanitarian community "to stay just one little tiny step ahead" while it rushed to build new camps and find spaces in existing camps for families returning to already liberated eastern Mosul.

Grande said that if more than 40,000 people flee at one time, no institution in the world can help, so the Iraqi military has made a real effort when they attack a neighborhood to keep the daily flight of people below the saturation point of 20,000.

"We're averaging between 8,000 and 15,000 a day," she said.

Grande stressed, however, that Islamic State fighters are making it extremely difficult to leave the Old City by surrounding it with concrete walls shaped like upside-down T's, and placing snipers all along the walls who are shooting "anyone who's trying to escape."

There have been more than 7,000 gunshot wounds of people trying to leave districts still controlled by the Islamic State group, also known as ISIL, she said.

"The reason we know that they're being shot at by snipers and not crossfire is because they're being shot in the back," Grande said.

This was discovered by one of the new innovations in the humanitarian operation in Mosul: "trauma stabilization units" that have been set up around IS-controlled areas to do "immediate triage of trauma victims" as soon as they cross into safety, which has helped save lives, she said.

These units, established by the World Health Organization, do immediate stabilization of patients, which usually takes about 20 minutes, Grande said. Patients are then put in an ambulance or another available vehicle to be transported to a field hospital or for very complicated cases to a trauma hospital in a major city.

She said another innovation, by the UN Population Fund, has been to deploy near the front lines specialized mobile teams to support fleeing women and girls who have been sexually abused. Typically, such units would be well back from conflict lines but having them close to the front has helped women and girls "who have experienced horrific conditions under ISIL occupation," Grande said.