The United Nations Children's Fund (UNICEF) warned of a potenial humanitarian disaster in Fallujah, Iraq Wednesday, estimating that 20,000 children were trapped with their families inside the ISIS-held city. 

In a statement, UNICEF called on Iraqi troops and ISIS militants to "protect children inside Fallujah" and "provide safe passage to those wishing to leave the city." The statement also warned that children ran the risk of being forced to fight alongside the terror group.

"Children who are forcibly recruited into the fighting see their lives and futures jeopardized as they are forced to carry and use arms, fighting an adults' war," UNICEF said.

Backed by aerial support from the U.S.-led coalition and paramilitary forces mainly made up of Shiite militias, Iraqi government troops more than a week ago launched a military operation to recapture Fallujah which has been under control of the extremist group for more than two years.

As the battled unfolded -- with Iraqi forces this week pushing into the city's southern sections after securing surrounding towns and villages -- more than 50,000 people are believed to be trapped inside the Sunni majority city, about 40 miles west of Baghdad.

Fallujah was the first large city in Iraq to fall to ISIS and it is the last major urban area controlled by the extremist group in western Iraq. The Sunni-led militants still control the country's second-largest city, Mosul, in the north, as well as smaller towns and patches of territory in the country's west and north.

The fight for Fallujah is expected to be protracted because ISIS has had more than two years to dig in. Hidden bombs are believed to be strewn throughout the city, and the presence of trapped civilians will limit the use of supporting airstrikes.

The Associated Press contributed to this report.