Camp Tariq, Iraq: Elite Iraqi special forces have begun their push into Fallujah, expecting to encounter the stiffest resistance yet in the campaign to free territory from the Islamic State group.

The city 65km west of Baghdad has been under militant control longer than any other part of Iraq, and IS fighters have had more than two years to dig in. Networks of tunnels like those found in other IS-held territory have already been discovered in its northeastern outskirts.

The Iraqi troops, also known as the counterterrorism forces, are leading the assault on Fallujah, slowly moving up from the southern edge in a column of armoured Humvees.

Their advance is expected to be slow because tens of thousands of civilians remain trapped in Fallujah and hidden bombs are believed to have been left throughout the city, according to special forces commanders at the scene. They expect fierce resistance from the jihadis, who have nowhere to run.