"Today, God willing, we start an important military campaign to liberate the citizens of Salahuddin province which includes Samarra, Dhuluiya, Balad, Dujail, al-Alam, al-Door, and Tikrit and other areas in the province from ISIL," Prime Minister Haider al-Abadi said on Monday.
"I call upon you and all other commanders to deal with citizens well. Our goal is to liberate people from the oppression and terrorism of Daesh," he added in a televised address, referring to the Arabic acronym used to describe ISIL fighters.
Some 30,000 Iraqi troops started attacking ISIL positions in Tikrit from the northern, western and southern fronts early on Monday, backed by artillery and air strikes by Iraqi fighter jets.
Iraqi authorities reported that security forces were in the process of retaking some neighborhoods in Tikrit's northeast.
Hospital sources told Al Jazeera that two members of popular mobilisation forces and one government soldier was killed on Tikrit's northern front on Monday, while 35 more were wounded.
Hours ahead of the operation, Abadi called on tribal fighters to abandon the group, promising them a pardon.
Tikrit, some 130km north of Baghdad, fell into the hands of ISIL last summer along with the country's second-largest city of Mosul.