Samaritan's Purse is working on the ground and says it is ready to aid people in Iraq's second largest city.
The city has been under IS control since June 2014 and tens of thousands of Christians have been forced to leave.
Hiruy Teka, International Disaster Response Programme Manager at Samaritan's Purse, told Premier's News Hour they have been preparing to help those that will be affected and need prayer.
He said: "We have been working to preposition supplies that will provide assistance to people, so our call is for churches across the UK to pray for the people of Mosul, and secondly for organisations, such as ourselves, that are gearing up to provide much-needed assistance at this time."
- Population pre-2014: around 700,000 people
- Tens of thousands have fled since 2014
- Red Cross says up to a million people are homeless
He warned that the numbers fleeing would be huge: "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."
Those 200,000 are expected to need aid within the first 48 hours of the attack.
"We have prepositions in place for food and water and medical equipment for tens of thousands of people," said Hiruy Teka.
"It's very difficult to say 'we know exactly what's going to happen, we know exactly what to expect', given our experience we are as prepared as we can be at this stage."
Details of the military offensive are secret, however progress is stalling due to arguments between Turkey and the Iraqi government about the plan.
Listen to Premier's Antony Bushfield speak to Hiruy Teka here: