Falluja was the first city to fall to IS in 2014 and is one of only two of its remaining strongholds in Iraq.
Iraqi Prime Minister Haider al-Abadi has announced the start of a military operation to retake Falluja from so-called Islamic State (IS).
Iraq’s military has already warned civilians to leave the town.
The Iraqi military told state TV that those who could not flee should raise a white flag above their homes.
The military, police and volunteer fighters virtually surround the city, about 65 km (40 miles) west of Baghdad.
The Baghdad-based pro-Kurdish news website Shafaq said on Sunday that close to 20,000 police troops had arrived on the outskirts of Falluja ahead of the expected assault.