Turkey and the United States have agreed on the outlines of a de facto “safe zone” along the Turkey-Syria border under the terms of a deal that is expected to significantly increase the scope and pace of the U.S.-led air war against the Islamic State in northern Syria, according to U.S. and Turkish officials.

The agreement includes a plan to drive the Islamic State out of a 68-mile-long area west of the Euphrates River and reaching into the province of Aleppo that would then come under the control of the Syrian opposition. If fully implemented, it would also bring American planes in regular, close proximity to bases, aircraft and air defenses operated by the Syrian government, and directly benefit opposition rebels fighting President Bashar al-Assad’s regime.

Operations in the targeted area would stop short of meeting long-standing Turkish demands for a full-scale, declared no-fly zone, but the area could eventually become a protected haven for some of the estimated 2­ million Syrian civilians who have fled to Turkey. […]