Dozens of people have been killed in a series of attacks in the northern Iraqi city of Tikrit, according to security sources.

The overnight attacks on Wednesday came after several suspected ISIL fighters infiltrated Tikrit, around 170km north of the capital, Baghdad.

The attackers, who reportedly wore police uniforms and used a police vehicle to enter the city, targeted a security checkpoint and the house of a police colonel, who was killed with four members of his family, according to officers.

Two suicide bombers detonated their vests when surrounded by police, and three others were killed in separate clashes.

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A total of 31 bodies were taken to hospital, including 14 belonging to policemen, Nawfal Mustafa, a doctor at the city's main hospital, told the Reuters news agency.

The death toll rose in the morning as more bodies were found, belonging to civilians killed in their shops.

The violence, which also left at least 42 people wounded, prompted Tikrit authorities to declare a curfew in the city. Sporadic gunfire could still be heard on Wednesday morning.

There was no immediate claim of responsibility but ISIL fighters have carried out similar attacks in Tikrit in the past, in an apparent diversionary tactic as Iraqi forces push ahead with a US-backed offensive to dislodge the group from the remaining districts under its control in Mosul, further north.

ISIL, or the Islamic state of Iraq and the Levant, seized Tikrit during a lightning offensive that overran large areas north and west of Baghdad in the summer of 2014, but Iraqi forces recaptured it the following year.