Humanitarian aid workers are expecting up to one million people -- including up to 600,000 children -- to flee the besieged city of Mosul as the long-awaited offensive to liberate the Islamic State stronghold pushes into its second day.

So far, there has been no mass exodus of civilians. Military officials have suggested ISIS fighters are holding the city’s population against their will in order to form a human shield to slow the U.S.-led coalition’s advance.

Alun McDonald, the regional media manager with Save The Children, tells CTV News Channel that while the fighting hasn’t yet reached the city, civilians plotting their escape are already facing an impossible choice.

“Get caught in the crossfire of the bombing that we are expecting in the coming days or … run the gauntlet of snipers, and landmines, and booby-traps that have been laid all around the city,” he said from Erbil on Tuesday. “Whether they stay or go they risk death.”

The charity is prepared to provide emergency water supplies, food and other items to displaced refugees when they begin to arrive en mass, but McDonald says such a large influx of people will quickly overwhelm their resources.

“We expect anything up to a million people to flee the city if the offensive escalates as predicted. At the moment, there are only really camps for about 80,000 people, which is obviously well short of what is likely to be needed,” he said.

McDonald’s says history suggests those who escape Mosul will be in need of long-term support. Save The Children operates across several camps in northern Iraq which are still housing people who fled the Mosul after it fell under ISIS control two years ago.

“They can’t go home because it’s too dangerous,” he said. “We are very worried if large numbers are displaced this time it could well be a similar amount of time before people can return home.”