Alleged Islamic State militants stand next to an ISIS flag atop a hill in the Syrian town of Ain al-Arab, called Kobane by the Kurds, as seen from the Turkish-Syrian border in Suruc, Turkey, on Monday. Aris Messinis/AFP/Getty Images hide caption

itoggle caption Aris Messinis/AFP/Getty Images

Fighters from the Islamic State group parade in their northern Syrian stronghold, Raqqa. Uncredited/AP/Raqqa Media Center hide caption

itoggle caption Uncredited/AP/Raqqa Media Center

Syrian rebel fighters in the northern city of Aleppo in August. The Obama administration has been vetting rebel groups and decided that more than a dozen are moderate enough to arm. Zein al-Rifai/AFP/Getty Images hide caption

itoggle caption Zein al-Rifai/AFP/Getty Images

This image, posted on a militant website, shows an Islamic State fighter waving a flag from a captured government fighter jet in Raqqa, Syria. The Islamic State captured the city in northeastern Syria last year and it has effectively served as its capital. Uncredited/AP hide caption

itoggle caption Uncredited/AP

Smoke rises from the Beiji oil refinery during clashes between the Islamic State and Iraqi government forces in Beiji, northern Iraq, on July 30. The militants tried to take the refinery this summer, but government forces have held on. STR/EPA/Landov hide caption

itoggle caption STR/EPA/Landov