A Russian official said on Saturday it would not be possible for an international conference aimed at bringing Syrian government and opposition envoys together for negotiations by the end of May.
U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov had announced on Tuesday that Washington and Moscow would seek to organise a conference on Syria and expressed hope it could be held this month.
"By the end of May is impossible," the Russian official, who was at talks on Friday between President Vladimir Putin and British Prime Minister David Cameron, told reporters on condition of anonymity.
Russian news agencies quoted the same official as saying there are disagreements over who should have the right to take part in a Syrian peace process, suggesting this could impede efforts to organise an international conference.
The official said there is broad agreement that the situation in Syria is dire but "beyond that there are very many differences: who can take part in this format, who is legitimate and who is not legitimate," state-run Itar-Tass reported.
Russia has opposed sanctions against Syria and blocked three U.N. Security Council resolutions hostile to President Bashar al-Assad, but insists it is not trying to prop him up and has backed calls for creation of a transitional government.