But American officials say that this plan is not directed against Mr. Assad. They also say that while a de facto safe zone could indeed be a byproduct of the plan, a formal no-fly zone is not part of the deal. They said it was not included in the surprise agreement reached last week to let American warplanes take off from Turkish air bases to attack Islamic State fighters in Syria, even though Turkey had long said it would give that permission only in exchange for a no-fly zone.

Instead, United States officials said Turks and Americans were working toward an agreement on the details of an operation to clear Islamic State militants from a heavily contested area roughly between the eastern outskirts of the city of Aleppo and the Euphrates River.

That is an ambitious military goal, because it appears to include areas of great strategic and symbolic importance to the Islamic State, and it could encompass areas that Syrian helicopters regularly bomb. If the zone goes 25 miles deep into Syria, as Turkish news outlets have reported, it could encompass the town of Dabiq, a significant place in the group’s apocalyptic theology, and Manbij, another stronghold. It could also include the Islamic State-held town of Al Bab, where barrel bombs dropped by Syrian aircraft have killed scores, including civilians, in recent weeks.

American officials emphasized that the depth of the buffer zone to be established was one of the important operational details that had yet to be decided. But one senior official said, “You can be assured many of the principal population centers will be covered.”

The plan does not envision Turkish ground troops entering Syria, although long-range artillery could be used across the border. Turkish ground forces would work on their side of the border to stem the Islamic State’s ability to infiltrate foreign fighters and supplies into Syria.

Turkey, a NATO member, has requested a meeting of the alliance on Tuesday about its fight against the Islamic State and Kurdish militants. NATO officials said Turkey had not requested major military support from the alliance. A number of members are already a part of the American-led coalition fighting the Islamic State.