A US occupation patrol in Mosul "responded to a drive-by shooting directed at an Iraqi police traffic control point by four assailants in a car and a motorcycle," an army statement said on Saturday.
"Working with the Iraqi police, the patrol engaged the car, killing two of the assailants," it said, adding that two suspects were detained.
But Lieutenant Colonel Abd al-Ajal Hazim Hattabi from the provincial police said the US army shot dead civilians who happened to be in a vehicle at the scene of the attack.
"Three men on a motorbike were preparing to attack the checkpoint with rocket-propelled grenades and automatic weapons. US forces opened fire and hit a car that was happened to be there," he told reporters.
In other operations in Mosul, the military said it detained four suspects, three after it found a homemade bomb in a search operation and one other suspected of "anti-coalition activities".
In the oil-rich city of Kirkuk, further south, attacks against Iraqi police claimed another victim early on Saturday.
"Unknown attackers in an Opel vehicle opened fire on a police roadblock this morning at 8:30 (O5:00 GMT) at the eastern entry to the city and police officer Ali Hamada was killed," said police chief Turhan Yusuf.
"We will pursue them and open an investigation," he said, without providing further details about the suspects.