Iraqi troops have retaken some districts around Tikrit in their fight to recapture the city from Islamic State (ISIS).
A force of about 30,000 soldiers and militia are said to be attacking on different fronts, backed by air strikes from Iraqi jets.
Tikrit, north of the capital Baghdad, fell to ISIS militants last June.
The troops had seized control of the two districts of al-Tin, near Tikrit university north-east of the city, and the district of al-Abeid, in the west.
Earlier, fighting was also reported in al-Dour, south-east of Tikrit, as well as in al-Alam, north of the city, and nearby Qadisiya.
There were few details of the operation but army and medical sources were quoted as saying that five soldiers and 11 militia fighters had been killed.
The Pentagon said that the US was not providing any air power in support of the operation.
Iraqi PM Haider al-Abadi declared the start of the operation late on March 1, as tens of thousands of troops and militia massed in the central town of Samarra.
Tikrit, in Salahuddin province, lies on the road to Mosul – Iraq’s second city which was also seized by ISIS last year.
Correspondents say the current operation is crucial to any Iraqi plans to retake Mosul.
Iraqi forces are being helped by Iran’s Gen. Qasem Soleimani, commander of the Revolutionary Guard’s Quds Force, Iranian and Iraqi media reported.
Since the ISIS advance across Iraq last summer, Gen. Qasem Soleimani has personally overseen the defense of Baghdad and helped to organize pro-Iranian Shia militia.
He has been pictured visiting the front lines north of the capital on several occasions.
Tikrit is the hometown of deposed leader Saddam Hussein and was seized last year by IS militants backed by anti-government Sunni allies loyal to Saddam’s banned Baath party.
Iraq is split between a Sunni Muslim minority, many of whom supported Saddam Hussein, and the Shia Muslim majority.
Since Saddam Hussein was toppled, Sunnis have felt increasingly marginalized by the Shia-led government in Baghdad.
Shia militia has done much of the fighting against ISIS militants but have also been accused of killing scores of Sunni civilians in apparent revenge attacks.
ISIS militants hold several areas of Salahuddin, a predominantly Sunni Muslim province.
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