The military operation came as the top Shiite cleric urged the country's leaders to unite, after Prime Minister Nuri al-Maliki conceded political measures are needed to defeat the jihadist-led offensive that has killed more than 1,000 people and overrun major parts of five provinces.
In further fallout from the crisis, the president of Iraq's autonomous Kurdish region declared there was no going back on Kurdish self-rule in disputed territory, including ethnically divided northern oil city Kirkuk, now defended against the militants by Kurdish fighters.
Iraqi forces swooped into Tikrit University by helicopter yesterday, and a police major said that there were periodic clashes on the campus today.
A senior army officer said Iraqi forces were targeting militants in Tikrit with air strikes to protect forces at the university and prepare for an assault on the city.
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Troops are deployed in areas around Tikrit for the attack, the officer said.
The military counter-offensive is the latest effort to regain the initiative after security forces wilted in the face of the initial insurgent onslaught, led by jihadists from the Islamic State of Iraq and the Levant (ISIL) but involving other groups as well, which began late on June 9.
Iraqi Kurdish leader Massud Barzani said Baghdad could no longer object to Kurdish self-rule in Kirkuk and other towns from which federal forces withdrew in the face of the militant advance.