The US Secretary of State says Russia's policy toward Syria will contribute to a potential civil war.
Hillary Clinton's comments came after Russia and China renewed opposition to tougher UN Security Council action.
UN Secretary General Ban Ki-moon has repeated a warning that Syria could be moving towards "catastrophic" civil war, in the wake of the Houla massacre.
Rebel commanders are split on whether to abandon a ceasefire if Syrian forces do not withdraw to barracks.
Colonel Qassim Saadeddine of the Free Syrian Army (FSA) in Homs said that if there was no government response by Friday lunchtime, the FSA would consider itself "no longer bound" by the plan.
But the FSA head, General Riyad Asaad, later denied the deadline existed.
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