Russia has harshly criticised the Arab League's recognition of the Syrian opposition as the only representative of the country, saying it effectively kills efforts to negotiate a peaceful end to the civil war in Syria.
Assad regime ally, Russia, also said on Thursday it would strongly oppose any bid to give Syria's UN seat to the opposition.
Russia's UN envoy Vitaly Churkin said recognising the Syrian National Council would "undercut the standing of the UN".
At a summit in Qatar on Tuesday, the Arab League let the main Syrian opposition coalition take over the Syria's seat for the first time.
"We perceived the results of the Arab League summit in Doha with regret. I will be frank about it. We see that the prevalent meaning of the decisions made there is that the Arab League has written off any peaceful settlement [in Syria]," Russian Foreign Minister Sergei Lavrov said at a news conference in Moscow.
He told reporters that the decision amounts to discarding of an international peace plan approved in Geneva in June, which was supported by the Arab League at the time.
It called for an open-ended cease-fire and peace talks to form a transitional government that would run the country until elections.
However, the plan was a non-starter for the opposition because of Moscow's insistence it did not explicitly ban Syrian President Bashar al-Assad and other members of his regime from taking part in the transitional leadership.
Lavrov said the summit's decision on Tuesday "strokes out all the efforts that have been made", including the Geneva agreements, and throws the status of Lakhdar Brahimi, UN and Arab League envoy for Syria, into limbo.
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"If one of the founders of his mission, the Arab League, declares that the opposition coalition is the only legitimate representative of the country, there will be no talks and those who want to oust the regime will be provided with weapons. I simply cannot see how Mr Brahimi could remain the envoy," Lavrov continued.
"That effectively puts an end to international mediation from the point of view of the Arab League."
Lavrov said the Doha summit's decision signalling an intention to supply the Syrian opposition with weapons is aimed at "inciting confrontation and encouraging irreconcilable forces".
Russia has been Assad's main supporter throughout the two-year conflict, joining forces with China at the UN Security Council to shield his regime from international sanctions over his crackdown on an uprising that turned into a civil war that has killed an estimated 70,000 people.