A passenger on the bus told Associated Press Television News (APTN) that the tank crashed into a bus in Tarmiyah, 50km north of Baghdad, late on Thursday, as they returned from a wedding.
"An American armoured vehicle hit them and left," he said, adding 12 people from the same family had been in the bus.
Iraqi police Captain Adnan Salih also said another 18 were injured as they were returning from a wedding party. He said the casualties included four men, four women and a child.
APTN footage showed the twisted wreckage of a minibus and several bodies of accident victims covered by blankets. A small child's orange dress was found amid the wreckage and pools of blood covered the road.
However, the US military gave a different version of the incident.
In a statement on Friday it said a van collided with a US tank and that nine were killed and 10 injured. There were no US casualties.