Doctors at a hospital in Aleppo, Syria, treat a boy injured in what the government said was a chemical weapons attack on March 19. Syria's government and rebels accused each other of firing a rocket loaded with chemical agents outside of Aleppo. George Ourfalian/Reuters/Landov hide caption

itoggle caption George Ourfalian/Reuters/Landov

The White House said this week that it believes Syria has been using chemical weapons, but President Obama has not said how the U.S. might respond. Here, rebel fighters in Syria prepare to launch of a rocket in the northern Syrian city of Aleppo, on April 21. The rebels have also accused the Syrian government of employing chemical weapons. Miguel Medina /AFP/Getty Images hide caption

itoggle caption Miguel Medina /AFP/Getty Images

A top Israeli military official on Tuesday accused Syrian President Bashar Assad's regime of using chemical weapons in recent fighting in Syria. Syria has denied using such weapons. Assad is shown here at the Opera House in Damascus in January. Anonymous/SANA/AP hide caption

itoggle caption Anonymous/SANA/AP