But others within the administration, and many outside experts, are increasingly worried that if President Obama does not take decisive action — such as quickly moving to claim the airspace over northwestern Syria and the Turkish border, where Russian jets are already operating — it is the United States that will suffer significant damage to both its reputation and its foreign policy and counterterrorism goals…

Recent efforts by the United Nations and others to organize new negotiations, beginning with localized cease-fires, have already begun to fall apart under Russian bombardment that is likely to change Assad’s calculus. Some see the Kremlin’s goal as maintaining Assad’s control over a rump Syrian state, in western population centers away from areas of Islamic State dominion, an objective that would also ensure Russia’s continued foothold in the Middle East.

“If [the possibility of negotiations] existed until one or two weeks ago, we definitely find ourselves today . . . in a completely different place,” said one international official who has long been involved in the effort. “We’ve been completely blown out of the water.”

A cease-fire reached with Iran and Hezbollah to allow aid and evacuation of civilians from Zabadani, west of Damascus near the Lebanese border, has now unraveled “because of Russian action,” the international official said. Although coordinates had been shared with Russia and plans were in place for U.N. officials to enter the area Saturday, it has been bombed three times, said the official, who like other U.S. and international officials spoke on the condition of anonymity to discuss internal deliberations.