The United States believes that at least five Syrian rebels who were trained by the U.S. military have been captured by suspected members of Syria's al-Qaida affiliate, Nusra Front, a U.S. official said on Tuesday, speaking on condition of anonymity.

If confirmed, the capture of the U.S.-trained fighters would be another blow to the U.S. military's still fledgling program to deploy local forces to counter Islamic State militants inside Syria, supporting them with U.S.-led coalition airstrikes.

U.S. President Barack Obama has ruled out deploying American ground forces to battle the group.