* Kremlin's special envoy points to rebel gains
* Moscow making preparations to evacuate Russians if needed
* Clearest sign Russia preparing for Assad's possible defeat
By Steve Gutterman
MOSCOW, Dec 13 (Reuters) - Rebel forces are gaining ground against the Syrian government and could win the war against President Bashar al-Assad, the Kremlin's envoy for Middle East affairs said on Thursday.
Deputy Foreign Minister Mikhail Bogdanov said Russia was working on plans to evacuate Russians 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," state-run Russian news agency RIA quoted him as saying.
"Unfortunately, a victory of the Syrian opposition cannot be ruled out," he said. "We are dealing with issues of preparations for an evacuation. We have mobilisation plans and are clarifying where our citizens are located."
His remarks were the clearest sign yet 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.
"The fighting will become even more intense and (Syria) will lose tens of thousands and, perhaps, hundreds of thousands of civilians," Bogdanov was quoted as saying by the state-run Russia Today broadcaster.
"If such a price for the removal of the president seems acceptable to you, what can we do? We, of course, consider it absolutely unacceptable."
Russia has shielded Assad's government from U.N. Security Council censure and sanctions, resisting Western pressure to join efforts to push him from power.
President Vladimir Putin and other Russian officials have repeatedly said Moscow is 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.
Bogdanov indicated Russia's stance would not change.
"Moscow will continue to insist upon the implementation of the Geneva communique and a peaceful resolution of the conflict," he said, according to RIA.
International envoy Lakhdar Brahimi, who has met Russian and U.S. officials twice in the past week, is seeking a solution based on an agreement reached in Geneva in June that called for the creation of a transitional government in Syria.
Bogdanov said Russia could meet Brahimi and U.S. officials again to support his efforts, but Moscow has warned that international recognition of a new opposition coalition, notably by the United States, is undermining diplomacy.
The United States says 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 realises that," said Fyodor Lukyanov, the well-connected 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 has been one of Moscow's most important footholds in the Middle East since the Soviet era, hosting a naval maintenance and supply facility that is Russia's only military base outside the former Soviet Union. Syria has 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 has warned the West it will not allow a repeat in Syria of last year's events in Libya, where NATO intervention, authorised by the U.N. Security Council after Russia abstained from a vote, helped rebels to topple Muammar Gaddafi.
But Russia has tried to distance itself from Assad and shown signs that it is positioning itself for the day he may lose power, not least by meeting with Syrian opposition groups.
A senior lawmaker allied to Putin said last week Assad's government was incapable of carrying out its functions.
There are about 5,300 Russian citizens registered with Russian consular authorities in Syria, but Bogdanov said the majority of Russians there were not registered. Many are Russian women married to Syrian men, and their children.