US Secretary of State Hillary Clinton says Russian policy will contribute to a potential civil war in Syria.
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.
The FSA’s Colonel Qassim Saadeddine 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.
Instead, he urged peace envoy Kofi Annan to issue a statement declaring his peace plan to have failed.
Syrian President Bashar al-Assad has come under intensified pressure to adhere to the ceasefire plan since the Houla massacre, in which more than 100 people – many of them children – died.
Hillary Clinton, speaking on a visit to Denmark, said the case for military intervention was growing stronger every day.
“[The Russians] are telling me they don’t want to see a civil war. I have been telling them their policy is going to help to contribute to a civil war,” she told an audience in Copenhagen.
Ban Ki-moon, speaking at a conference in Turkey, said UN monitors had not been sent to Syria “just to bear witness to the slaughter of innocents”.
“We are not there to play the role of passive observer to unspeakable atrocities,” he said.
“The massacre of civilians of the sort seen last weekend could plunge Syria into catastrophic civil war – a civil war from which the country would never recover.”
Colonel Qassim Saadeddine’s ultimatum, citing the Houla massacre, was given in a video released online, in which he said the government had to “implement an immediate ceasefire, withdraw its troops, tanks and artillery from Syrian cities and villages”.
“It should also allow immediate humanitarian aid to all affected areas and free all detainees… The regime should also enter into a real and serious negotiation through the United Nations to hand over power to the Syrian people,” he went on.
But General Riyad Asaad, speaking to al-Jazeera by phone from Turkey, insisted the FSA was “committed to the Kofi Annan plan and committed to international resolutions and implementing this plan”.
“There is no deadline; however, we hope that Kofi Annan will issue a statement to announce the failure of this plan,” he said.
The UN Human Rights Council will hold a special session on Syria on Friday looking into the killings, officials said.
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.