A Russian official has said it is "impossible" to organise an international conference by the end of this month which eventually aims to bring together the Syrian government and opposition for negotiations.
Washington and Moscow announced joint plans earlier this week for such a summit, which would work towards ending the country's bloody civil war.
The Russian official, who was at yesterday's talks between President Vladimir Putin and Prime Minister David Cameron, said: "By the end of May is impossible".
The anonymous official said disagreements remained about who could take place in the peace process.
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