U.S. is debating ways to shield Syrian civilians

Nonetheless, the fact that the administration is even revisiting an idea it has previously rejected — just weeks after Mr. Obama publicly dismissed it again — underscored the urgency of the crisis as tens of thousands of Syrians flood Europe to escape the war zone and Russian airstrikes fuel the multisided conflict. It also suggested a frustration on the part of policy makers seeking a strategy that can succeed.

Among the options discussed on Monday were establishing safe zones for civilians on Syria’s borders with Turkey and Jordan. Officials presented different variants, including some that had safe zones exclusively for humanitarian relief and more ambitious versions that would provide sanctuary for Syrian opposition forces allied with Americans.

But the Pentagon presentation laid out how many aircraft and personnel would be required, making it clear that there would have to be a significant escalation of American air power in the region, according to officials who described private deliberations on the condition of anonymity. The officials said that the escalation would require aircraft and personnel beyond those already conducting airstrikes against the Islamic State in Syria and Iraq.