"Russia's military intervention has dangerously exacerbated an already complex environment," Anne Patterson, the US assistant secretary of state for Near Eastern affairs, told the House Foreign Affairs Committee.
Patterson said the Russian air strikes have so far targeted moderate opponents to Syria's President Bashar al-Assad, killing civilians in attacks on civil defense crews, hospitals, centers for displaced persons and ambulances.
"So far, then, this has not been a Russian fight against terrorism so much as an effort to preserve the Assad regime," she said.
But she added that the United States should undertake a "full court press to end the war and get to a political settlement."
"Regardless of progress towards a political transition, it is our position that given its new involvement in the military situation, Russia has an even greater responsibility to stop immediately the regime's horrific practice of barrel bombing and use of chlorine gas against its population.