Iraq's top Shiite cleric meanwhile 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.
A senior American official said that the US military was flying "a few" armed drones over Baghdad to defend American troops and diplomats in the city if necessary.
But officials said the drones would not be used for offensive strikes against the Sunni Arab militant offensive, led by jihadists from the Islamic State of Iraq and the Levant (ISIL) but involving other groups as well.
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.
Troops are deployed in areas around Tikrit for the attack, the officer said.
Another senior officer said taking the university is an important step towards regaining control of Tikrit, the hometown of executed dictator Saddam Hussein, which the militants seized on June 11.
The operation is the latest effort to regain the initiative after security forces wilted in the face of the initial insurgent onslaught launched on June 9.