As Iraqi troops battles ISIS for control of Fallujah, the U.N. children's fund warns at least 20,000 children are trapped inside.

The northern Iraqi city has been under Islamic State control for more than two years. A week ago, Iraqi government troops launched an operation to re-capture it.

There are concerns that ISIS is using children and their families as human shields, a not uncommon tactic among Islamic terror groups, such as Hezbollah in Lebanon and Hamas in the Gaza Strip.

More than 50,000 civilians are believed to be trapped in Fallujah, many of them children who face not only a dire humanitarian situation but are also at risk of being forced to fight for the Islamic State.

"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," the children's fund said in a statement. It called on "all parties to protect children inside Fallujah" and "provide safe passage to those wishing to leave the city."

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.

Many expect a drawn-out fight for Fallujah because ISIS has had a long time to dig in.

There is also concern the group has planted bombs throughout the city, and the presence of civilians will limit the use of airstrikes.

