The world is watching. The use of chemical weapons is and would be totally unacceptable," said Mr Obama in a speech at the National Defense University in Washington.
Two security staff at the hall and a press photographer were also injured. Separately, trouble broke out on the Lower Newtownards Road in east Belfast.
It marks a change to the current policy by which the flag is displayed outside the building 365 days a year. The council's decision has angered many loyalists.
The Press Association (PA) also said that one of its photographers had sustained a head injury after getting caught up in the violence at the city hall.