Aircraft have struck at rebel positions and clashes have broken out in various parts of Tikrit, witnesses and officials have said.
Troops had reportedly pulled back to the nearby town of Dijla as Saturday’s initial offensive met stiff resistance.
The city of Tikrit was captured by Sunni rebels on June 11 as they swept across large parts of northern Iraq.
“The security forces are advancing from different areas,” Lt-Gen. Qassem Atta told journalists.
“There are ongoing clashes.”
Insurgents, led by the Islamic State in Iraq and the Levant (ISIS), were reported to have shot down a helicopter and captured the pilot.
The witnesses said the Iraqi forces had been hampered in their bid to retake Tikrit by the large number of improvised explosive devices laid on the approaches to the city.
Iraq said on Sunday it had received the first batch of military jets ordered from Russia in order to help fight the militants.
The defense ministry said five Sukhoi aircraft would enter service in “three to four days”.
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