Iraqi forces say they are continuing to push into the Islamic State-held city of Fallujah, having secured its outskirts.

Iraqi officers said Thursday that the large number of civilians in the city was slowing down the military operation, because it means fewer airstrikes can be carried out.

Brig. Haider al-Obeidi says "there were fewer civilians in Ramadi, therefore airstrikes are now less than during Ramadi operation."

Iraqi forces are heavily dependent on airstrikes to achieve territorial victories against IS.

The U.S.-led coalition says they have carried out four airstrikes against IS targets in and around Fallujah since Wednesday. The strikes hit fighting positions, a weapons cache and a tunnel system.

Fallujah is one of the last IS strongholds in Iraq. The militants also hold Mosul, the second largest city.