"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.
She said Russia's intervention made Moscow all the more responsible for getting Syria to end its use of barrel bombs and chlorine gas against its people.
Civilians also have been killed in Russian air strikes, she said, citing attacks on civil defense crews, hospitals, centers for displaced persons and ambulances.
"Since the beginning of Russian strikes in Syria, at least 120,000 Syrians have been displaced as a result of regime offensives aided by Russian air strikes in the cities of Hama, Aleppo, and Idlib," she said.
Victoria Nuland, the assistant secretary of state for European affairs, told the same committee that since the start of the Russian air strikes, Greece has recorded its highest weekly inflow of refugees and migrants from Turkey, some 48,000.
Patterson repeated US charges that the Russian air campaign has targeted moderate opponents of Syria's President Bashar al-Assad, rather than the Islamic State jihadists a US-led coalition has been fighting.
Washington has previously estimated that 90 percent of the Russian air strikes were not targeting IS forces but other rebel groups fighting the Assad regime.
"So far, then, this has not been a Russian fight against terrorism so much as an effort to preserve the Assad regime," Patterson said.
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"Neither we nor the Russians know exactly what effect the Russian action will have over the military balance on the ground –- the results so far have been mixed, as the opposition puts up a very strong fight," she said.
- 'Full court press' -
But she added that the situation called for a "full court press to end the war and get to a political settlement."
"The Russian deployments cannot be used to stiffen the Assad regime's resistance to a political transition," Patterson said.
Since the end of October, the United States and Russia have been engaged in a diplomatic effort in Vienna to try to lay out the outlines of a political transition in Syria.
But participants in the talks have clashing views over Assad's fate, with Washington demanding he step down and Moscow saying he should remain in power for now.
"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.
"The international community looks to Russia to take up that responsibility," Patterson said.
The United States has waged a separate air campaign against the Islamic State in Syria and Iraq for more than a year, but has had little measurable impact on the Sunni extremist group.
It was taken by surprise by the Russian military intervention at the end of September, which Moscow cast as a complementary fight against "terrorism."