Reuters quotes VP and Rafik Hariri Center Director Francis Ricciardone on Turkish support of extremist organizations in Syria: 

Francis Ricciardone, who was until late June the U.S. ambassador in Turkey, said on Thursday Ankara had supported groups including the Nusra Front, al Qaeda’s Syrian branch.

“We ultimately had no choice but to agree to disagree,” Ricciardone told a conference call arranged by the Atlantic Council think-tank on Thursday. “The Turks frankly worked with groups for a period, including al Nusra, whom we finally designated as we’re not willing to work with.” 

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