The United States and Turkey have not yet agreed which Syrian rebels they will support in a joint effort to help clear Islamic State from an area along the Turkish border, officials said on Tuesday, underscoring uncertainty surrounding the campaign plan.

Washington and Ankara this week announced their intention to provide air cover for Syrian rebels and jointly sweep Islamic State fighters from a strip of land along the border, with U.S. warplanes using bases in Turkey for strikes.

But planning appears to be just getting under way and resolving crucial details, like which opposition groups will be supported on the ground, could stoke longstanding tensions between the United States and Turkey about Syria strategy. […]