BEIRUT — Turkey and the United States have agreed on the outlines of a plan to rout the Islamic State group from a strip of Syrian territory along the Turkish border — a plan that opens the possibility of a safe haven for tens of thousands of displaced Syrians but one that also sets up a potential conflict with U.S.-backed Syrian Kurdish forces in the area. The move further embroils Turkey, a key NATO ally, in Syria’s civil war, and also catapults it into a front-line position in the global war against the Islamic State. A senior Obama administration official said Monday that U.S.