A senior officer said security forces was coordinating with the US, which has deployed military advisers to help the government push back the militants, who have overrun large parts of five provinces north and west of Baghdad.
Armed US drones were flying over Baghdad to provide protection for the advisers and US diplomats against the militants, led by the jihadist Islamic State of Iraq and the Levant (ISIL).
International agencies have raised alarm bells over the humanitarian consequences of the fighting, with up to 10,000 people having fled a northern Christian town in recent days and 1.2 million displaced by unrest in Iraq this year.
Thousands of soldiers, backed by air cover, tanks and bomb disposal units, were advancing on Tikrit - now-executed dictator Saddam Hussein's hometown - which fell to insurgents on June 11.
Witnesses said loyalists were moving toward Tikrit from the west and engaged in heavy clashes.
Helicopter-borne troops swooped into a strategically located university campus in the city on Thursday, with sporadic clashes reported throughout Friday.
Taking the university is seen as an important step towards regaining control of Tikrit, one of the biggest cities held by the militants.
Iraqi forces were carrying out air strikes against insurgents inside the city, Prime Minister Nuri al-Maliki's security spokesman said.
They were also now in full control of a key road from Baghdad to Samarra, between the capital and Tikrit, Lieutenant General Qassem Atta added.
Although they initially wilted in the face of the offensive in majority Sunni Arab areas that began on June 9, the security forces have appeared to perform more capably in recent days.
