Islamic State fighters blew up a key bridge on Friday in Iraq's Salaheddin province as they retreated in the face of an offensive by pro-government forces, officials said.

Iraqi troops, Kurdish peshmerga forces and Shiite militias have been pushing west from the city of Tuz Khurmatu since Thursday.

"At 5:00 am (0200 GMT), peshmerga and Iraqi forces attacked several areas, going from Tuz Khurmatu towards Tikrit, to reach Zerga bridge, a strategic link between the two," Mullah Karim Shukur, a senior official in the Patriotic Union of Kurdistan Party, said.

"During this advance, retreating IS forces blew up the bridge and left dozens of bodies behind," he said.

Shallal Abdul, the mayor of Tuz Khurmatu, said five villages had been wrested back from the jihadists on Thursday and four more on Friday before reaching the bridge over a tributary of the Tigris.

"They are going to retake all the areas under IS control, they fled leaving bodies behind," he told AFP.

Tikrit, executed former president Saddam Hussein's hometown, has been under IS control since the first days of the major offensive the jihadists launched in Iraq four months ago.

The Iraqi army has tried and failed to recapture it several times.

Security sources said warplanes carried out strikes on IS targets north of Tuz Khurmatu, near Taza, on Friday.

A senior police officer from Kirkuk province said 16 jihadists were killed in the strike, although the figure could not be immediately verified.