Iraqi state television reported Tuesday that the attack occurred that day in the Sunni insurgent stronghold west of Baghdad. Iraqi police and military confirmed the account, but later said the bombing took place Monday.
The report brought denunciations from top Iraqi officials and international groups about violence targeting children.
But Rear Adm. Mark Fox, a U.S. military spokesman, said "the allegation was false" and suggested that rumors began circulating after a controlled detonation by U.S. forces caused injuries in Ramadi.
On Tuesday, a military statement said 30 civilians and one Iraqi soldier were injured by flying debris when troops destroyed 15 bags of explosives. None of the injuries was life-threatening, it added, reports AP.
"There was no second blast," Fox told reporters, "and there was no 18 children killed."
Iraqi officials have made no further comment on the report about the car bombing.