According to the Iraqi media reports, the military forces began the operation at 3 a.m. local time from three directions.
Tikrit, about 150 km (95 miles) north of Baghdad, is the hometown of executed dictator Saddam Hussein.
Iraqi soldiers and voluntary forces have been massing for days in preparation for the attack on the ISIL strongholds along the Tigris River to the north and south of Tikrit.
The ISIL militants made advances in northern and western Iraq in summer 2014, after capturing swaths of northern Syria.
However, a combination of concentrated attacks by the Iraqi military and the volunteer forces, who rushed to take arms after Ayatollah Sistani issued the fatwa calling for fight against the militants, have blunted the edge of the ISIL offensive.