The violence has killed about 40,000 people.
Deputy Foreign Minister, Mikhail Bogdanov, also said Russia was working on plans to evacuate its citizens from Syria if necessary.
“One must look the facts in the face. The regime and government in Syria is losing control of more and more territory,” the state-run Russian News Agency, RIA, quoted Mr. Bogdanov.
“Unfortunately, a victory of the Syrian opposition cannot be ruled out,” he said.
His remarks were the clearest sign yet that Russia is preparing for the possible defeat of Assad’s government in a conflict that has killed more than 40,000 people since March 2011.
“We are dealing with issues of preparations for an evacuation. We have mobilization plans and are clarifying where our citizens are located,” Mr. Bogdanov said.
Russia had shielded Assad’s government from UN Security Council censure and sanctions, resisting Western pressure to join efforts to push him from power.
President Vladimir Putin and other Russian officials had said that Moscow was not trying to prop up Assad but that he must not be ousted from power by external forces, citing the principle of non-interference in sovereign states’ affairs.
Mr. Bogdanov indicated Russia’s stance would not change.
“Moscow will continue to insist upon the implementation of the Geneva communiqué and a peaceful resolution of the conflict,” he said, according to RIA.
International Envoy, Lakhdar Brahimi, who had met Russian and U.S. officials twice in the past week, was seeking a solution based on an agreement reached in Geneva in June that called for the creation of a transitional government in Syria.
Mr. Bogdanov said Russia could meet with Mr. Brahimi and U.S. officials again to support his efforts, but Moscow had warned that international recognition of a new opposition coalition, notably by the U.S., is undermining diplomacy.
The U.S. said the Geneva agreement sent a clear message that Assad should quit, but Russia contends that it did nothing of the kind.
“Time is working against Assad and Russia realizes that,” said Fyodor Lukyanov, the editor of the journal Russia in Global Affairs.
“I think a diplomatic solution is no longer possible and, since the world would not let Assad win, the opposition assumes sooner or later it will seize power by force,” he said.
Syria had been one of Moscow’s most important footholds in the Middle East since the Soviet era, hosting a naval maintenance and supply facility, Russia’s only military base outside the former Soviet Union.
Syria has also been a major buyer of Russian arms.
But analysts say Russia’s refusal to budge on Assad is driven by Putin’s distaste for U.S.-led intervention abroad.
Moscow had warned the West that it would not allow a repeat in Syria of last year’s events in Libya, where NATO intervention, authorized by the U.N. Security Council after Russia abstained from a vote, helped rebels to topple Muammar Gaddafi.