Just two days after American F-16 jets rescued U.S.-trained rebels under attack in Syria, at least five more have been kidnapped and another killed, a defense official told The Daily Beast. The trainees reportedly came under attack near the Turkish border. It marks the deadliest attack on what is essentially the U.S. ground force in the war against the ISIS terror group.

There are no more than 60 Syrian fighters trained by the U.S. and they only stepped onto the battlefield a few weeks ago. Over the weekend, U.S. jets carried out the first strikes to protect trainees who came under fire, leading the Obama administration to declare it would defend U.S.-trained fighters, known as the New Syrian Forces, from attacks by either ISIS or the al Nusra Front, an al Qeada-affiliated group.

The Pentagon said Tuesday it could no longer assure all the rebels can be accounted for. It is a recent about-face: On Friday, spokesman Navy Capt. Jeff Davis said the 54 fighters were all accounted for. On Monday, the U.S. military admitted it conducted airstrikes over the weekend to provide assistance to U.S.-trained fighters who came under attack from al Nusra. On Tuesday, Davis said he could no longer assert the U.S. knew the whereabouts of its trainees but refused to provide details, even as reports emerged that at least one U.S.-trained fighter had been killed.

— Nancy A. Youssef