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"In our view, the United States is definitely not working hard enough in terms of putting influence on Syrian opposition groups so that (they) will come to the international conference," state run RIA quoted Ryabkov as saying.
He said the United States "should not allow the opposition to try to issue ultimatums and impose preconditions. The main such condition. is the demand for Syrian President Bashar al-Assad's exit."
The opposition Syrian National Coalition said late last month it would only take part in the peace talks if a deadline was set for a settlement that would force Assad to leave power.
Russia has been Assad's most powerful protector during the conflict that has killed more than 80,000 people since March 2011, opposition sanctions and, with China, vetoing three U.N Security Council resolutions aimed to pressure his government.
(Writing by Steve Gutterman: editing by Elizabeth Piper) (Reuters) (GNN)