WASHINGTON - The United States welcomed on Thursday a Russian admission that Syria's rebels may succeed in their drive to topple President Bashar Assad and called on Moscow to join efforts to manage a peaceful political transition.

"We want to commend the Russian government for finally waking up to the reality and acknowledging that the regime's days are numbered," State Department spokeswoman Victoria Nuland told a news briefing.

"The question now is, will the Russian government join those of us in the international community who are working with the opposition to try to have a smooth democratic transition?"

Russian Deputy Foreign Minister Mikhail Bogdanov, the Kremlin's envoy for the Middle East, said on Thursday that rebel gains on the ground mean that their ultimate victory over Assad "cannot be ruled out.".

Bogdanov's comments were among the most pessimistic yet from Russia, which has shielded Assad's government from UN Security Council censure and sanctions, resisting Western pressure to join efforts to push him from power.

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