Reuters news agency reporters say they've seen at least 15 bodies near three badly damaged apartment buildings in a residential area of the city.
Some local people have said the number of dead could be as high as 45.
Coalition forces are trying to hurt as few civilians as possible so if this is confirmed this is a big blow to them.
A spokesman for the Royal Air Force said that an investigation would be held into the incident.
On Tuesday night military targets and Iraq's main TV station were hit. The station couldn't broadcast anything for three hours.