The city has been under Islamic State control since 2014. The militant group ordered believers to either pay a tax, leave or face death.
Mosul is famous for being the spiritual home of the Assyrian Church and the resting place of major Old Testament prophets like Jonah.
There are up to 40,000 troops attempting to recapture the city backed by aerial bombing from coalition forces.
There are around 4500 Islamic State fighters inside Mosul. The militant group has used civilians as human shields in the past and it's thought they've booby-trapped entire streets with mines making entering the city extremely dangerous.
Christian charity World Vision has raised concern for the families stuck in the city.
Khalil Sleiman, Response Manager for northern Iraq, said: "We're already supporting half a million people who fled Mosul when it was first occupied over two years ago.
"We're now poised for another massive influx of children and families who will have been through horrific experiences most of us could never imagine.
"The violence will also have taken a devastating emotional toll on children, many of whom will need years of specialist support to rebuild their lives, come to terms with what happened, and to regain some kind of normality.
"Children always bear the brunt of conflict and we call for humane treatment at every stage of the process of the Mosul operation - including screening when boys as young as 14 may be separated from their families."
Kurdish Peshmerga fighters stand on top of a military vehicle as they advance towards villages surrounding Mosul, in Khazer, about 30 kilometers (19 miles) east of Mosul
Sarmad Ozam is a Christian refugee who left the city with his family.
He told Premier he was worried for his friends left behind: "The people will be killed as well. Not just IS. Now the army will come and they will target IS and the people as well.
"It's always the people that die from these wars."
Retired Major General Tim Cross told Premier's News Hour the city could be left in ruins: "If they have to fight over this, the bottom line is the Iraqi military will basically take every building down, otherwise they are going to take huge number of causalities.
"If it develops into IS actually saying, we're going to stand and fight, there will be suicide bombers and IEDs [bombs] all over the place.
"This is not a conventional battle with people in uniform".
