WASHINGTON • Already out- gunned and out-manned in Syria's civil war, US-backed rebels are facing a new and possibly more serious threat to their survival: Russian air strikes that Washington appears reluctant to thwart.
The Obama administration - blindsided by the speed of Moscow's direct intervention and a Russian target list that included CIA-trained fighters - made clear on Thursday that it had no desire to increase the risk of an air clash between the former Cold War foes.
While Washington took pains to insist it still considered the "moderate" opposition vital to Syria's future and was not abandoning them, withholding US air cover could further jeopardise beleaguered rebel forces.
President Barack Obama has rarely launched military action in support of the opposition in four years of Syria's civil war and is hesitant to get further ensnared in the conflict.
Even if he wanted to, he could face legal limitations due to the scope of his presidential war powers.
The rebels have already struggled in the fight against the Syrian military, dogged by internal divisions and the rise of radical militant groups such as Islamic State in Iraq and Syria (ISIS) and the Al-Qaeda- linked Nusra Front.
Russian warplanes, in a second day of strikes on Thursday, bombed a camp run by anti-government rebels trained by the CIA, the group's commander said, even as Russia insisted it was hitting only ISIS forces, a common enemy of Washington and Moscow.
US officials believe Moscow's main objective is to prop up its ally, Syrian President Bashar al-Assad.
Russia's deepening role, together with inconclusive talks between the US and Russian militaries on air safety on Thursday, underscored the consensus in Washington that Mr Obama has few good options for turning the situation around.
Mr Obama does have the power to expand the arming of moderate rebels, so they can better defend themselves, or to set up no-fly zones, as some critics at home have demanded, but US officials note that such measures would carry their own risks of escalating Washington's involvement.
Russian President Vladimir Putin appears to be betting that Mr Obama, wary of seeing the United States pulled into another Middle East war, would be unlikely to respond aggressively.
"Mr Putin reads the Obama administration well," wrote Mr Aaron David Miller, a former Middle East adviser to Democratic and Republican administrations. "He knows that President Barack Obama never wanted to militarise the US role in Syria."