The doubts by Damascus's most important ally followed remarks in which the international envoy for Syria, Lakhdar Brahimi, said the target date of mid-November was "not 100% certain" and cited disunity among rebel forces.
Russian Foreign Minister Sergei Lavrov said: "Until recently we hoped our Western partners, who undertook to bring the opposition to the conference, could do it quite quickly, but they were unable to do it quickly, and I don't know whether they will be able to do so by mid-November."
A pledge by the Syrian government to abandon chemical arms has boosted prospects for the peace conference, proposed by Russia and the US in May, to go ahead.
UN Security Council powers hope it can be held in mid-November. Mr Lavrov said it must be organised soon because "radicals and jihadists are strengthening their positions" in Syria.
He added: "The task is to not lose any more time, and to bring to the negotiating table with the government those opposition groups that think not about just seizing power and using it at their will, but about the fate of their country."