Eighteen gas pipeline workers, most of them Iranians, have been shot dead north-east of Baghdad, officials say.
Security sources confirmed to the BBC that the 15 Iranians and three Iraqis had been working on a gas pipeline to run between their two countries.
The attack happened near the city of Baquba. The sources said it took place in a predominantly Sunni area.
Another five Iranians and two Iraqis were wounded in the attack, the sources added.
The UN has said the monthly death toll dropped to 659 in November, including 565 civilians and 94 members of the Iraqi security forces, compared with 979 in October.
But more than 7,150 civilians and 950 security forces personnel have now been killed since January, the highest annual toll since 2008.