washington u.s. secretary state rex tillerson meet senior turkish officials ankara next week talk could vital advance u.s.-backed campaign retake syrian city raqqa islamic state u.s. officials say friday turkey press unite state drop military alliance syrian kurdish ypg militia consider part kurdistan workers party fight insurgency three decades turkey u.s. officials long view kurdish fighters key approach assault raqqa islamic state de facto capital would work alongside arab fighters u.s.-backed syrian democratic force sdf instead turkey want unite state draw syrian arab rebel group back ankara final assault raqqa predominantly arab city proposals far fail convince u.s. officials certain turkish-backed arab force large well-trained enough first need work detail turkey one senior official say speak condition anonymity decision set president donald trump wish quick battlefield victories need maintain unite state longstanding strategic alliance turkey nato ally provide unite state access base critical air war syria raqqa campaign appear gather steam overlap u.s.-backed effort iraq draw close drive islamic state city mosul french defense minister jean-yves le drian say friday battle raqqa would likely start come days head syrian kurdish ypg militia tell reuters last week assault would begin start april ypg would take part u.s. officials think timeline optimistic note major battle underway tabqa dam 25 miles 40 km west raqqa could take weeks complete pentagon also say unite state decide composition american-backed assault force raqqa unite state 1,000 troop syria moment raqqa campaign could involve hundreds senator john mccain senate arm service committee chairman suggest reporters early week would hard convince turkish president tayyip erdogan merit back kurdish fighters even effective weapon islamic state conundrum use kurds take lot long islamic state send stream terrorists raqqa mccain say enormous challenge far relations turkey concern add mccain say relationship erdogan particularly important cite april 16 referendum constitutional change extend power tillerson meet ankara also closely follow meet 68 coalition partner week washington agree intensify fight islamic state include syria wednesday meet tillerson say unite state would create interim zone stability ceasefires help refugees return home next phase fight islamic state al qaeda syria iraq elaborate although one u.s. official say yet agreement among american agencies zone would set u.s. military long skeptical create safe zone inside syria iraq partly safe zone guarantee unite state almost certainly would require american military protection secure grind alone would require thousands troop former u.s. officials experts say top u.s. commander grind lieutenant general stephen townsend say early march build safe zone right iraqis syrians safe islamic state increase u.s. ally air power would require trump choose enforce fly restrictions grind force might also need protect civilians areas state department provide detail tillerson trip turkey say visit brussels march 31 participate reschedule meet nato foreign minister nato meet set april 5-6 reschedule tillerson decide instead attend summit china cause unease among european ally already worry trump commitment western military alliance tillerson visit turkey raqqa operation heat