Aid Groups Prep for Humanitarian Crisis After Battle for Mosul A million people could be forced to flee their homes.

 -- As the military operation to retake the Iraqi city of Mosul from ISIS begins, U.N. groups and aid agencies are preparing for a complex humanitarian disaster that could affect up to 1.5 million people, with civilians forced to either flee their homes as refugees or get caught in the crossfire and be used as "human shields."

Civilians in Mosul

Up to 1.5 million people living in Mosul are expected to be affected by the military operation, according to Stephen O'Brien, the U.N.'s undersecretary-general for humanitarian affairs and emergency relief coordinator.

"Families are at extreme risk of being caught in crossfire or targeted by snipers. Tens of thousands of Iraqi girls, boys, women and men may be under siege or held as human shields," he said in a statement last night.

Save the Children, an aid group on the ground in Iraq, estimates that there are 500,000 to 600,000 children trapped in the city.

"Unless safe routes to escape the fighting are established, many families will have no choice but to stay and risk being killed by crossfire or bombardment, trapped beyond the reach of humanitarian aid with little food or medical care," Aram Shakaram, the deputy country director in Iraq for Save the Children, said in a statement.

"Those that try to flee will be forced to navigate a city ringed with booby traps, snipers and hidden land mines. Without immediate action to ensure people can flee safely, we are likely to see bloodshed of civilians on a massive scale," he added.

Alun McDonald, an aid worker with Save the Children in Erbil, Iraq, told ABC News today that he worries there is not enough being done to protect civilians in Mosul as the military operation gets underway.

"At the moment, the security advice has been to stay at home and to put white flags on their buildings" to signal to fighters not to attack, he told ABC News.

He said, "In fairness to the military operation, it is incredibly difficult to secure a safe route out of Mosul. But, I mean, putting a white flag on your building is not really going to keep you safe on the battlefields. It is quite likely that soldiers, if they want to protect themselves, will wave white flags, so in terms of keeping people safe, it is not really going to help."

McDonald added that he is concerned about the major lack of funding and resources available address the impending humanitarian crisis for the people of Mosul.

"I think there are so many millions or billions of dollars put into military actions and military offenses, but getting money to deal with the fallout of those offenses is always more difficult," he said. "The money tends to be steered more toward the military side than toward the humanitarian side of things."

Aleksandar Milutinovic, the International Rescue Committee's Iraq director, said in a statement today, "Civilians who attempt to escape the city will have little choice but to take their lives into their own hands and pray that they are able to avoid snipers, land mines, booby traps and other explosives."

Refugee Crisis

Officials predict an exodus of people to flee what has become ISIS' de facto capital in Iraq in the coming weeks after the offensive.

As many as 200,000 could flee during the first weeks of fighting, and as many as 1 million could flee in a "worst-case scenario," according to the U.N. Office for the Coordination of Humanitarian Affairs.

Meanwhile, shelter is currently available for only 60,000 people in camps and emergency sites outside Mosul, according to the OCHA.

McDonald, who has been helping coordinate Save the Children's relief efforts on the ground, said they have seen people fleeing in large numbers from areas around Mosul before the offensive, and over the next two days the group expects to start seeing masses of people coming out of Mosul.

"We've seen in the past week or 10 days about 5,000 people fleeing from Mosul into Syria," he said. "Normally, we see people coming out of Syria, but this time we are seeing them cross the border into Syria. This gives you an idea of how desperate people are to leave [Mosul] if they can."