As Iraqi forces battle Islamic State (IS) militants for control of Mosul, World Vision says it's caring for more than 21,000 civilians who have fled the city so far.
World Vision director Alex Snary says the charity is expecting at least 200,000 people to head to their camps, and around 3.3 million people in the area could be affected by the conflict.
"It's hard to imagine what it's like when the normal rules of law are suspended," Mr Snary told Paul Henry. "You don't know how to keep yourself safe.
"Where do you get access to food? Where do you get access to water? How do you survive?"
IS has held Mosul for more than two years, and the Iraqi army began targeting the city in late October.
In the last few days the conflict has entered the city and Mr Snary says it's moving house-to-house, with civilians trapped and looking for somewhere safe to hide.
"I know from my own experience of working in these sort of areas, when the bombs start to drop and the shelling starts, it's absolutely terrifying," he says.
The people heading to World Vision's camps outside Mosul have already had a tough time living in the IS-occupied city for the past two years.
They have to risk their lives to get to safety, and are likely to be shot at trying to escape the city, Mr Snary says.
"You've got children who are separated from their parents, you've got people who have literally grabbed what they could as they ran out the door of their house."
"It's hard to imagine the impact that this has on people, particularly on children.
"The children go through this process - initially they're crying and they're screaming because it's terrifying, but then they start to shut down. It's like they've cried all their tears they've got, and they begin to withdraw into themselves."
World Vision offers a "child-friendly" area of the camp, which Mr Snary says is like a classroom setting.
"If we can't help those kids to return to a state of normality, then they're going to carry that on into the future, and they're going to be very young, angry kids in the future."
Mr Snary says World Vision was already working to help more than half a million people before this latest crisis.