United States President Barack Obama expressed concern about Russia's military action in support of the Syrian regime.
He urged Moscow to cooperate with Washington to find a way out of the crisis.
"The US remains prepared to work with Russia to try to reduce the violence and strengthen our efforts against ISIL, Al-Qaeda and Syria," Obama told reporters at the Pentagon, using an alternate acronym for the IS jihadist group.
"But Russia has failed to take the necessary steps. Given the deteriorating situation, it is time for Russia to show it is serious about pursuing these objectives," Obama said, following a meeting with his National Security Council.
Russia and the United States back opposing sides in Syria's five-year war, which has left 280,000 people dead and forced half the population to flee their homes.
Multiple rounds of international negotiations to end the war, which erupted in 2011 after the Assad regime unleashed a brutal crackdown against a pro-democracy revolt, have so far failed.
Russia, an ally of Damascus, and the United States, which supports moderate rebels fighting to unseat President Bashar al-Assad, are nominally co-chairs of the international effort to bring the Syrian regime and armed opposition groups to the negotiating table as a step toward ending the war.
"I'm not confident that we can trust the Russians and (President) Vladimir Putin, which is why we have to test whether or not we can get an actual cessation of hostilities that includes an end to the kinds of aerial bombing and civilian death and destruction that we have seen carried out by the Assad regime," Obama said.
"We go into this without any blinders on."
Following heavy Russian air strikes on the outskirts of divided Aleppo city, US Secretary of State John Kerry on Monday urged Russia to "restrain" itself and its ally in Damascus from "offensive operations."