An international aid organization says 12,000 people have fled the outskirts of Fallujah since the operation to retake the Iraqi city from the Islamic State group was announced nearly two weeks ago.

The Norwegian Refugee Council said Friday that almost all the fleeing families lived on the city's outskirts.

Iraq's prime minister says safe corridors were established to allow some civilians to flee, but residents inside the city and aid groups say almost all of the 50,000 civilians inside Fallujah are trapped.

Fallujah has been held by IS extremists for more than two years. Iraqi forces have retaken most of the territory around the city, but are facing heavy resistance while pushing toward the center.