ISTANBUL — With the United States continuing to pressure Turkey to do more in the fight against the Islamic State, Turkey’s position has hardened around an idea it has pushed for years as a strategy to confront the chaos of the Syrian civil war: a buffer zone along its frontier with Syria.

The idea is emerging as a possible way to end the standoff between the United States and Turkey, and American military planners are said to be looking at how to implement such a plan, which would require a no-fly zone and stepped up combat air patrols to take out Syrian air defense systems.

Yet the prospect of a buffer zone is proving deeply divisive in Washington, as it would go far beyond President Obama’s original mission of taking on the Islamic State and would lead to a direct confrontation with the Syrian government of President Bashar al-Assad.

While Turkey has largely described the plan in humanitarian terms — to protect refugees and also Turkey’s border — the argument made privately is that a buffer zone would quickly evolve into a place where moderate rebels would be trained to fight Mr. Assad’s government; in other words, a fledgling rebel state.