WASHINGTON—U.S. and Turkish officials have narrowed their differences over a joint military mission in Syria that would give the U.S. and its coalition partners permission to use Turkish air bases to launch strike operations against Islamic State targets across northern Syria, according to officials in both countries.

As part of the deal, U.S. and Turkish officials are discussing the creation of a protected zone along a portion of the Syrian border that would be off-limits to Assad regime aircraft and would provide sanctuary...