The US is expected to push for tougher sanctions
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She said Moscow and Beijing had nothing to gain by standing up for Mr Assad, and their actions were intolerable.
More than 100 countries, known as Friends of Syria, are making a fresh attempt to decide what should be done to end the violence in Syria.
Russia and China are not attending.
On Thursday it emerged a senior member of Syria's regime had deserted it and fled to Turkey.
The pro-government website Syriasteps said Brig Gen Manaf Tlas had made an "escape" but that this was "insignificant".
Sources close to the family confirmed his departure to the BBC. But it is not clear whether he has defected to the opposition.
If confirmed it would be the highest-level defection of a supporter of President Assad since the rebellion against him started 16 months ago.
Earlier rumours about his possible defection in March proved to be false.
Brig Gen Tlas, of the Republican Guard, is the son of a former Syrian defence minister. As a young man he attended military training with Mr Assad.