dancing in the aisles in tehran and isis camps is they just don't believe in dancing. this is a big gift to them, and this is a devastating decision for our allies. >> reporter: one u.s. defense official telling cnn operational commanders are concerned about the future for syrian fighters backed by the u.s. of whom 1,600 have already been killed. if the u.s. pulls out the kurdish fighters face a potential bloodbath from both isis and a turkish invasion, and the isis threat is likely to skyrocket with thousands of suspected isis operatives now in custody, possibly getting released to fight again. >> we have to be clear the highest concentration of jihadists anywhere in the world in syria. >> reporter: general joseph dunford, chairman of the joint chiefs and general otell, head of middle east operations, are now talking to allies in the