BEIRUT — Syria’s most powerful ally, Russia, said today that President Bashar Assad is losing control of his country and the rebels might win the civil war, the first time Moscow has acknowledged the regime is cracking under the force of a rebellion.

Russian Deputy Foreign Minister Mikhail Bogdanov also said Moscow is preparing to evacuate thousands of its citizens from Syria, where nearly two years of violent conflict have killed more than 40,000 people.

“An opposition victory can’t be excluded, unfortunately, but it’s necessary to look at the facts: There is a trend for the government to progressively lose control over an increasing part of the territory,” Bogdanov said.

Bogdanov’s statement marks a clear attempt by the Kremlin to begin positioning itself for Assad’s eventual defeat at a time when rebels are making significant gains. Opposition fighters have seized large swaths of territory in north Syria and appear to be expanding their control outside of Damascus, pushing the fight closer to the seat of Assad’s power in the capital. On Wednesday, the U.S., Europe and their allies recognized the newly reorganized opposition leadership, giving it a stamp of credibility.

At the same time, international condemnation of the regime has grown as Western officials raise concerns that Assad might unleash chemical weapons against rebels in an act of desperation. On Wednesday, the U.S. and NATO said Assad’s forces had fired Scud missiles at rebel areas.

Fyodor Lukyanov, the editor of the magazine Russia in Global Affairs, said Bogdanov’s statement may reflect new information.

“A public statement like that appears to indicate that the balance is shifting,” he said.

Despite Russia’s acknowledgement that Assad could lose, Bogdanov gave no signal that Russia would change its pro-Syria stance at the U.N. Security Council, where Moscow has shielded Damascus from world sanctions.