"We pray for the swift recovery of all the shooting victims, and our hearts go out to their families," he said in a statement. "This senseless violence must be stopped."
The January shooting death of another child, 15-year-old Hadiya Pendleton, brought national attention to Chicago because the band majorette had performed in Washington at events surrounding President Barack Obama's inauguration in January.
The president invited Pendleton's parents to his State of the Union address in February. Obama mentioned her in that speech and a few days later when he returned to Chicago, his hometown, for another speech in which he pressed for stricter gun laws.
Police charged two men in Pendleton's death. They said the two were gang members seeking revenge and mistook Pendleton for someone else.
The FBI's annual crime report this week showed Chicago had 500 homicides in 2012, up from 431 in 2011 and more than any other American city. Chicago officials have said homicides this year are below the 2012 pace.
Still, Chicago does not have the highest homicide rate in the United States. That unwanted distinction belongs to Flint, Michigan, with one homicide for every 1,613 residents, according to the FBI data. Detroit is close behind, with one killing for every 1,832 residents.
In 2012, the number of violent crimes increased by 0.7%, compared with the previous year, according to the FBI figures.
Chicago bans some semiautomatic weapons and high-capacity magazines, and restricts handguns.