Russia says the Arab League's recognition of the Syrian opposition as the only representative of the country effectively kills efforts to negotiate a peaceful end to the civil war there.
Foreign Minister Sergei Lavrov said Thursday that Russia "deeply regrets" the move and was concerned about its consequences.
The Arab League let the main Syrian opposition coalition take over the country's seat for the first time at a summit Tuesday in Qatar.
Lavrov says the move amounts to a rejection of an international peace plan approved in Geneva in June, which was supported by the Arab League at the time.
He also argued that the move put the status of UN and Arab League envoy for Syria, Lakhdar Brahimi, in limbo.