MOSCOW — The outlook for Syria’s embattled president darkened considerably 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 ­Assad, was losing politically as well as militarily.