Mosul has been under IS control since June 2014 and tens of thousands of Christians have been forced to leave.
Speaking before the offensive by Iraqi and Turkish troops, Hiruy Teka, International Disaster Response Programme Manager at Samaritan's Purse, told Premier they have been preparing to help those that will be affected.
He said: "Our expectation is that in total anywhere between 600,000 to 700,000 people will need immediate assistance.
"Our understanding is, the minute the offensive begins, some 200,000 people will need basic food, water, basic shelter."
World Watch Monitor says sources on the ground say Bartella, a town with a large Christian population, are back in Iraqi government hands after two years.
The town, located east of Mosul, still needs to be cleared of mines and other explosives.
Bartella had a population of around 30,000 people before IS took over.
The region had been home to the largest concentration of Christians in Iraq.
Over the weekend IS militants stormed parts of a northern city, Kirkuk, in what is thought to have been a move aimed at diverting attention from Mosul, where Iraqi forces are waging a major offensive against IS.
There is an ongoing battle for another town outside of Mosul, Qaraqosh.
It used to have Iraq's largest Christian community and was considered one of the oldest communities of believers in the world.
A number of Christian towns are still awaiting liberation and Iraqi troops are still facing pockets of resistance in the region.
