Daesh attacked Iraq's largest oil refinery Thursday, reports AFP.
Clashes have been ongoing since Daesh launched its offensive at dawn.
The refinery is located near Baiji, located approximately 120 miles north of Baghdad. It had previously been besieged by Daesh militants last June when the militants claimed large swathes of land in northern Iraq.
Iraqi forces an allied Shiite militias have been fighting to regain control of Iraq's north. They have since had a substantial victory in reclaiming the city of Tikrit from the militants.
However, this might be one of the more challenging battles to date. "The battle for Baiji refinery is a real test for the Iraqi forces and it's one of the most complicated battles we have," an Iraqi army major general told AFP.
The US-led coalition claims to have conducted 22 airstrikes on Baiji in the month of May alone.
