The three men two Pakistanis and one Turkish were killed when unknown assailants opened fire on their vehicles on Wednesday, said Iraqi police.
The US military confirmed that two foreign nationals had been killed. A spokeswoman said one US soldier, a driver of an unspecified nationality and one US civilian were also injured.
Police said the incident took place 15km south of Tikrit.
Earlier, there were reports of an ambush on a convoy operated by US contractor Kellogg, Brown and Root. Two drivers were reported killed while several others were injured.
It was also revealed on Wednesday that eight Iraqis were killed on Tuesday in an apparent firefight with US soldiers near the town of Samarra, north of Baghdad.
US military officials accused the Iraqis of opening fire on occupation troops in a "drive-by shooting".
Military sources said 26 alleged attackers were apprehended following the incident and four vehicles were confiscated.