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ISIS launched a fresh offensive Thursday against Iraq's largest refinery, where a military official said security forces are facing one of their toughest battles, AFP reported.

ISIS launched "a fierce attack at dawn and clashes are ongoing," an army major general told AFP, on condition of anonymity because he is not authorised to talk to the press.

The country's largest refinery is located near the city of Baiji, around 200 kilometers (120 miles) north of Baghdad.

Following a victory against jihadi fighters in Tikrit, government and allied forces have vowed to continue working their way north. Control of the Baiji area is seen as key step toward the reconquest of the main ISIS hub of Mosul.

"The battle for Baiji refinery is a real test for the Iraqi forces and it's one of the most complicated battles we have," the major general said.

The vast refinery complex remained besieged by ISIS fighters for months following the group's massive onslaught across Iraq in June 2014.