Coalition and Iraqi planes have struck a sprawling complex of palaces in the city of Tikrit where Islamic State (IS) militants have been holding out for more than three weeks.

US-led coalition planes launched their first air strikes against IState targets in Saddam Hussein's home city late on Wednesday, coming off the sidelines to aid Iraqi forces fighting alongside Iran-backed Shi'ite militia on the ground.

The decision to provide air support for the Tikrit campaign draws the US into a messy battle that puts the coalition, however reluctantly, on the same side of a fight as Iranian-backed militia.

"The Iraqi air force with the coalition air force have conducted air strikes targeting the presidential palaces that are the headquarters of IS leaders and groups," Defence Ministry spokesman Brigadier Tahseen Ibrahim Sadiq said at an air base in Baghdad.

"This is the fourth sortie for the Iraqi air force since the early morning."

More than 20,000 Iraqi troops and allied Shi'ite paramilitary groups known as Hashid Shaabi have been taking part in the offensive since early March, but stalled around two weeks ago after sustaining heavy casualties.

Iraqi forces retook the area surrounding Tikrit in the first week of the campaign, and entered some districts of the city itself, which had been overrun in June by IS.

The militants have laid homemade bombs and booby traps in several areas including the presidential compound, which was built under Saddam and covers a huge area overlooking the Tigris river.

The mayor of Tikrit said coalition and Iraqi planes were also striking the northern Qadisiya district, part of which is still held by insurgents, as well as the Shisheen neighbourhood in the south.

Targets had to be carefully identified because IS fighters were believed to be holding prisoners in some of the 65 palaces.