Kidnappers in Syria have freed a U.S. journalist missing since 2012, U.S. Secretary of State John Kerry said on Sunday, following what Qatari-owned Al Jazeera said were efforts by the Gulf Arab state to win his release.

Kerry said in a statement announcing the release of Theo Curtis that the United States was using "every diplomatic, intelligence and military tool" at its disposal to secure the release of other Americans held hostage in Syria.

News of Curtis's release emerged just days after Islamic State militants this week killed American journalist James Foley, who was kidnapped in Syria in 2012.