REUTERS/Alaa Al-Marjani
A member of the Iraqi security forces fires artillery during clashes with Islamic State militants near Falluja, Iraq, May 29, 2016.
The city, located roughly 69 kilometers west of Baghdad, dates back to Babylonian times but was seized by militants in 2014, trapping tens of thousands of people.
According to the AFP, forces entered Fallujah from three directions early Monday morning, backed by tanks and artillery.
Lieutenant General Abdelwahab al-Saadi, the commander in charge of the operation, told reporters: "Iraqi forces entered Fallujah under air cover from the international coalition, the Iraqi air force, and army aviation."
He added that there has been "resistance" from the 500 to 1,000 fighters thought to be controlling the city.
The Independent reports that since the operation began on 23 May, troops have managed to reclaim 80% of the territory surrounding the ISIS stronghold, according to Iraqi Major Dhia Thamir.
Alaa Al-Marjani/REUTERS
Under ISIS' regime, anyone caught trying to flee its territory can be executed, effectively trapping up to 50,000 people within Fallujah. Previous reports suggested 15 people were burned to death for trying to leave.
Fallujah is one of the largest urban areas held by ISIS, along with Mosul - another city which has been the target of coalition-lead airstrikes. Fallujah had a population of more than 310,000 in 2010, but that number plummeted in the years leading up to the capture as conflict and food and medicine shortages drove out many residents.