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 in Homs said that if there was no government response by lunchtime today, the FSA would consider itself "no longer bound" by the plan.
The ceasefire exists in name only with most people within Syria believing that the UN plan will not work anyway.
The UN Human Rights Council will hold a special session on Syria today looking into the killings.
As many as 15,000 people have been killed since the revolt against the rule of President Bashar al-Assad began in March of last year.