BEIRUT - Hundreds of Syrian rebels are preparing to launch an operation to capture a town held by Islamic State at the border with Turkey, a senior Syrian rebel said on Sunday, a move that would frustrate Kurdish hopes to expand further in that area.The rebels, Turkey-backed groups fighting under the banner of the Free Syrian Army, are expected to assault Jarablus from inside Turkey in the next few days said the rebel official, who is familiar with the plans but declined to be identified.