At least 23 people have been killed in a string of attacks in northern Iraq, with several soldiers among the victims.

Twelve people were killed in a succession of car bombings in the oil-rich city of Kirkuk, about 250 kilometres north of Baghdad, police sources said.

The deaths included seven police recruits killed when a car bomb targeted them near an oil company northwest of Kirkuk.

More than 50 people were injured in the attacks, police said.

Elsewhere, 11 soldiers were killed and eight injured overnight when gunmen attacked an army checkpoint in the province of Salah al-Din about 170km north of Baghdad.

Iraq has seen a recent increase in such attacks, raising fears of a return to the violence that peaked in 2006 and 2007, when thousands were killed in sectarian violence.

At least 325 people were killed in attacks in Iraq in July alone, making it the highest monthly toll since August 2010, according to government figures.