Rancho Mirage, California (CNN) President Barack Obama urged Russian President Vladimir Putin to end his air campaign against Syrian opposition forces during a phone call Saturday, the White House said, a day after Putin's deputy described relations between Moscow and Washington sinking to Cold War depths.

In a description of the phone call, the White House said Obama stressed "the importance of rapidly implementing humanitarian access to besieged areas of Syria and initiating a nationwide cessation of hostilities."

Russia entered the Syria conflict in September, launching air strikes on forces aligned against Syrian President Bashar al-Assad from an airbase in Latakia province. U.S. officials have decried the strategy, saying Putin is desperate to keep Assad in power since Russia's sole remaining military base in the Middle East lies near Syria's Mediterranean coast.

The U.S. says Russia's air campaign has killed scores of civilians and jeopardizes the prospects of a peace accord in the civil war-wracked nation.

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