beirut u.s.-backed syrian democratic force sdf lead combatant islamic state thursday accuse turkey pressure one senior commanders defect rebel officials say wednesday brigadier general talal silo defect without give reason would first departure sdf top rank comment turkey back syrian rebel factions view sdf spearhead kurdish ypg militia security threat silo serve spokesman sdf alliance mostly kurdish arab militias battle islamic state syria help u.s.-led coalition sdf say statement lose contact silo resign post general talal silo respect appreciate among rank say subject lot pressure extortion side turkish state amount point threaten sons turkey believe disappearance result special operation turkish intelligence collusion family members kurdish fighters alongside arab ally u.s. advisers coalition air strike drive islamic state swathe territory include former headquarter raqqa city ypg ally carve autonomous canton north control nearly quarter syria influence anger neighbor turkey consider ypg extension ban kurdistan workers party fight decades-long insurgency turkish soil u.s.-led coalition say aware report talal silo apparent departure sdf detail current status time force affect incident inform public result investigation sdf say thursday u.s.-backed syria force say turkey pressure commander defect