MOSCOW — The outlook for Syria’s embattled president darkened considerably on Thursday when his most powerful foreign ally, Russia, acknowledged that he was losing the struggle against an increasingly coordinated insurgency and for the first time said it was making contingency plans to evacuate its citizens from the country, the Kremlin’s last beachhead in the Middle East.

The Russian assessment, made publicly by a top Foreign Ministry official in Moscow, appeared to signal a major turn in the diplomacy of the nearly two-year-old conflict and presented new evidence that the Syrian president, Bashar al-Assad, was losing politically as well as militarily. On Wednesday it was revealed that Mr. Assad’s forces had resorted to firing Scud ballistic missiles at rebels in an attempt to slow the insurgency’s momentum.

The assessment suggested that Russia no longer viewed Mr. Assad’s involvement in a negotiated solution as a viable alternative. It also appeared to reflect a new recognition in Moscow that Mr. Assad and his minority Alawite government, long a Russian client, could not survive in the face of a well-armed opposition financed by Arab and Western countries seeking his ouster. Some Russian officials have bitterly concluded that Mr. Assad’s foreign adversaries want an outcome decided by military force.