The Iraqi army says it has launched a major offensive to retake Tikrit from Sunni rebels, amid conflicting claims over who controls the city's university.

The main ground operation, which began on Saturday, followed heavy fighting in the city between Sunni rebels and Iraqi special forces, who were trying to establish a foothold at the university campus.

Zaid al-Ali, an Iraqi author and former UN adviser discusses politics in Iraq. 

Iraqi military sources said they had captured the university but rebels led by the Islamic State in Iraq and the Levant said they had successfully repelled the attack.

Al Jazeera's Imran Khan, reporting from Baghdad, said Iraqi armoured divisions have been moving towards Tikrit from Samarra, a nearby city where government forces had been massing. 

The Iraqi army said Tikrit had been "cleansed" of ISIL fighters, a statement denied by websites sympathetic to ISIL. Al Jazeera cannot verify these claims. 

The Iraqi army also said it had destroyed a convoy of about 20 rebel vehicles between Samarra and Tikrit.

ISIL supporters used social media to say the rebels had destroyed at least 10 humvees, six tanks and a helicopter in the clashes, and killed up to 300 soldiers.

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The operation is the largest the Iraqi army has undertaken in rebel-held territory, Khan said. 

US assistance

Meanwhile, the US said that armed drones were flying over Baghdad. Officials said the sorites were being flown to protect US military advisers helping Iraqi forces tackle the rebellion.

The defence department said the unmanned aircraft would watch over US troops who operated outside the confines of the US embassy, the AP news agency reported.

There were also reports of air attacks in Mosul, Iraq's second largest city in the hands of Sunni rebels since the ISIL launched lightning attacks earlier this month.

The US has said that targeting rebel leaders in air attacks is one option being prepared to deal with the rebellion that has taken huge parts of the country.

"We're building a picture so that if the decision were made to support the Iraqi security forces as they confront ISIL, we could do so," said the chairman of the Joint Chiefs of Staff, Martin Dempsey.

Source: Al Jazeera and agencies