President Obama spoke with Russian President Vladimir Putin to call for an end to airstrikes against moderate opposition forces in Syria as a security conference in Munich discussed efforts to implement a truce in the five-year civil war.

Obama, in Saturday's phone call, stressed the need for quick humanitarian access to besieged areas and a nationwide cessation of hostilities, the White House said in a statement released Sunday. He emphasized the importance of "Russia playing a constructive role by ceasing its air campaign against moderate opposition forces."

The Kremlin said earlier in a statement that Putin emphasized the importance of a united anti-terror front and close contacts between Russian and U.S. defense forces. The leaders agreed on strengthening diplomatic cooperation, according to the Kremlin.

The call was made after both countries' top diplomats cast doubt over their plan for a Syrian truce less than a day after it was agreed. Russian Foreign Minister Sergei Lavrov gave the deal less than a 50 percent chance of success. U.S. Secretary of State John Kerry said the U.S. and Russian militaries still needed to work out coordination that would allow strikes on Syrian terrorist groups without targeting the "legitimate opposition."

Outside powers in Syria's conflict, including the U.S., Russia, Turkey, Saudi Arabia and Iran, last week backed a truce set to start this Friday. Peace efforts have intensified after nearly six months of Russian bombing in support of Syrian President Bashar Assad. The Syrian regime, supported by Iranian and Hezbollah fighters, is threatening to drive rebels from Aleppo, once the country's biggest city and an opposition stronghold.

Earlier at the three-day Munich Security Conference, Kerry warned Russia to stop bombing opponents of Assad that aren't jihadists.

Nicholas Burns, a former U.S. undersecretary of state in the Bush administration, said he expects Russia to continue its air campaign until all supply lines to Aleppo are cut. "That seems to be one of their objectives," he said in an interview in Munich. "And it's a very cynical objective to have."