There were conflicting reports as to how far the military advanced in its initial thrust towards the northern city. (Agencies)
Residents said militants were still in control of the city by nightfall on Saturday, while Iraqi security officials said troops had reached the outskirts and the provincial governor said they had pushed into the city itself.
The government has presented the campaign as a rebound following weeks of demoralizing defeats at the hands of insurgents led by the al-Qaeda breakaway Islamic State of Iraq and the Levant.
The militants' surge across much of northern and western Iraq has thrown the country into its deepest crisis since US troops withdrew in December 2011, and threatens to cleave the nation in three along sectarian and ethnic lines.
If successful, the Tikrit operation could help restore a degree of faith in the security forces, as well as embattled Prime Minister Nouri al-Maliki, who is fighting to keep his job.
On Saturday fighting began before dawn with helicopter gunships carrying out airstrikes on insurgents who were attacking troops at a university campus on Tikrit's northern outskirts, Iraqi military spokesman Lt Gen Qassim al-Moussawi said.
The government forces had established a bridgehead on the university's sprawling grounds after being airlifted in the previous day.
There were conflicting reports as to how far the military advanced in its initial thrust towards the northern city.