Iraqi forces backed by foreign air power are fighting their way slowly and cautiously into Mosul. Islamic State overran the city in June 2014 with barely a shot fired, heralding the rise of an Islamist dictatorship responsible for some of the worst human rights outrages in modern times. Half a million people fled, including the Christian population of at least 70,000.

Islamic State is in its end-game, but it's had two years to prepare for it. There are tunnels, booby-traps and weapons caches. There are only a few thousand of them, but the damage they can do is terrible – and it's the civilians who will pay the heaviest price. So the 1.5 million remaining in the city are getting out as fast as they can.

Matt Nowery is the country director for Samaritan's Purse in Iraq. The organisation has been in the country since 2008, and it's now trying to help the Mosul refugees by providing food rations, shelter, clean water and cooking stoves. Speaking to Christian Today, he paints a vivid and heartbreaking picture of what's happening.

"Over the last week, there have been thousands of refugees. There's a full range of needs," he says.

"If you could picture several kilometres of cars with military escorts, every one with a white flag to show they're a civilian. There are dump trucks with a hundred people aboard. They are in shock."

Many others have been walking for hours and arrive at the camps exhausted. And it's not just their physical state: many of them are emotionally traumatised.

"When ISIS took over the city they were split down the middle," says Nowery. "Families were split and not able to contact each other. There are hundreds of family members looking in every car and dump truck to see if they can recognise someone. When they recognise someone there's such a range of emotions, it's hard to put it into words.

"Others are angry, screaming abuse – they don't understand what's happening. Many are confused. Others feel fear, they don't trust anyone, they don't know anyone or anything."

During the last week, numbers have risen to more than 40,000, double what they were before the assault began. Samaritan's Purse is helping provide blankets and other equipment to help refugees endure the winter, which can be bitterly cold in the region. But Nowery says no system could absorb the amount of people potentially displaced as the assault on Mosul continues: "There's no way we could meet all those needs."

Mosul, Iraq's second city, was also the heartland of the Christian population in the north of the country. When ISIS took it over, the Christians were forced to flee. Almost all of those leaving city now are Muslims, but Nowery stresses that Samaritans Purse serves those in need without distinction. "We are there to provide physical aid. It's an opportunity for us as an organisation to serve amid some of the worst trauma the world has seen," he says.

"I believe the people coming out of those villages and the city will remember who helped them in their hour of need. We're there to show the love of Jesus and to be his hands and feet."

And, he says, Samaritan's Purse is there for the long haul.

"It's a long, long job. We have been into the liberated villages and there is massive destruction. ISIS has been busy destroying infrastructure. They have torn out wiring, destroyed churches, water storage tanks. Rebuilding the villages is going to take a lot of work and a lot of time."

But he recounts a conversation with an 84-year-old priest displaced from Mosul he describes as a "God moment" for him. "He said, 'It may seem like a very dark place and that Islam is winning, but if you believe the same as me, you know who has the victory.'

"We have been here since the beginning and we have no plans to leave," he says. "We're going to help rebuild. We want to help provide education and health. This is a critical time and Christians are fleeing – Samaritan's Purse wants to put its arm round them and remind them Jesus loves them, and figure out what we can do to help them get on their feet."

Samaritan's Purse UK has urged Christians to pray for Mosul as it comes under attack.