At least 40 others were wounded in the blasts, which went off as worshippers were attending Friday prayers in the town of Khanaquin.
The blasts occurred several hours after two car bombs exploded in Baghdad, killing at least six people and injuring some 40 others.
The Baghdad blasts also took place near an Interior Ministry building where U.S. soldiers last week said they found some 170 prisoners, many of them showing signs of malnourishment and torture.
Iraqi authorities have promised to investigate. On 17 November, Interior Minister Bayan Jabor said the allegations had been exaggerated, and that only a handful of detainees showed signs of abuse.