The incident took place at the building site in Shirqat, a town in Salahuddin province 300 km (180 miles) north of Baghdad, the source said, and gave no further details.

Violence in Iraq has subsided since the peak of sectarian conflict in 2006/7, but dozens of bombings still take place each month. Kidnappers have held foreigners and Iraqis for ransom, and authorities say funds raised help fund the insurgency.