The United States lashed Moscow for arming the Syrian government and warned of the threat of civil war, as it stepped up diplomatic pressure on President Bashar al-Assad.
US Secretary of State Hillary Clinton, speaking in Denmark, said that Russia's policy of resisting UN Security Council action against Damascus was only increasing the chance of civil war erupting.
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," Clinton said.
At the United Nations, US Ambassador Susan Rice described Russian arms shipments to Syria as "reprehensible" while accusing Damascus of a "blatant lie" by denying involvement in a massacre in which 108 people were killed.
Their comments came after UN chief Ban Ki-moon warned that Syria risked a "catastrophic civil war" following the massacre in the village of Houla last week in which 49 children were killed.
As Syrian rebels threatened to escalate their operations unless Assad's regime meets a 0900 GMT Friday ultimatum to observe a UN-backed ceasefire, activists called for nationwide protests.
The rallies will honour the children killed in Houla, activists said on the Syrian Revolution 2011 Facebook page, describing them as "flares of victory".
China and Arab nations meanwhile jointly urged Syria's warring parties to fully implement a UN and Arab League ceasefire plan, which includes a halt in fighting that should have taken effect on April 12 but has been violated daily.
Implementing the plan "is essential to avoid the danger of a foreign intervention and the scenario of anarchy and civil war", the countries said in a statement issued after talks in the eastern Tunisian city of Hammamet.
"The international community must redouble its efforts and exhort the Syrian authorities to strictly and immediately apply the Annan plan," Chinese Foreign Minister Yang Jiechi said.
Speaking in Istanbul, UN chief Ban said Damascus must implement the six-point peace blueprint brokered by Kofi Annan.
"The massacres of the sort seen last weekend could plunge Syria into a catastrophic civil war, a civil war from which the country would never recover."
The UN says some of the 108 people killed in the central Syrian town of Houla last Friday and Saturday were killed by artillery tank fire.
But the UN High Commissioner for Human Rights says most were summarily executed, and some UN officials have said there were suspicions of involvement by pro-government shabiha militiamen.
Damascus on Thursday insisted a preliminary investigation showed "armed gangs" carried out the Houla killings.
"It appears that all the victims came from peaceful families who refused to rise up against the government or take up arms, but had rows with armed groups," said General Kassem Jamal Sleiman, head of the probe.
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Syrian foreign ministry spokesman Jihad Makdisi dismissed Ban's warning.
"It is regrettable that the Secretary General of the United Nations has departed from his mission of maintaining peace and security in the world to becoming a herald of civil war," Makdisi told reporters.
Rice's condemnation of Moscow's "reprehensible" arms deliveries to Syria came after reports last week that a Russian ship carrying weapons had arrived in the Syrian port of Tartus.
The ship, the Professor Katsman, apparently turned off its transponder on May 26 in the vicinity of Tartus, Sadia Hameed of the Human Rights First group told AFP. The vessel had been tracked from Piraeus in Greece.
Hameed said she could not be sure of the cargo because there was no official manifest. "The sense we get is that (the ship's contents) are small arms and ammunition."
Rice said: "With respect to the reported docking of a ship carrying Russian arms, this is obviously of the utmost concern given that the Syrian government continues to use deadly force against civilians."
Russia's UN ambassador Vitaly Churkin has rejected criticism of the arms sales insisting they are legal and have no influence on the Syria conflict.
The rebel Free Syrian Army's command inside the country, meanwhile, gave Assad's government an ultimatum of midday on Friday (0900 GMT).
If the regime "does not meet the deadline by Friday midday, the command... will no longer be tied by any commitment to the Annan plan... and our duty will be... to defend civilians," a statement said.
The FSA said it would announce in the coming days "a series of decisive and courageous decisions for the next phase" of the conflict.
The political opposition, the Syrian National Council, meanwhile urged Annan to raise the number of UN truce observers in Syria from their present number of 300 to 3,000.
The Houla massacre has prompted Western governments, including the United States, Britain, France and Australia, to expel the senior Syrian diplomats in their countries.
As the international community mulled fresh moves against Syria, US Defense Secretary Leon Panetta said any military action in Syria would need backing from the United Nations, but called recent violence "intolerable".
Asked if he could foresee a scenario in which the United States would back military intervention even without UN authorisation, Panetta said: "No, I cannot envision that."
President Vladimir Putin is expected to face pressure for Russia to drop its resistance to UN action on Syria when he meets the leaders of two of Europe's key powers on Friday.
Putin is due to hold talks with German Chancellor Angela Merkel and new French President Francois Hollande as part of his first foreign visit after returning to the Kremlin for a historic third term.
On the ground, Syrian forces resumed shelling in Houla, which had begun on Wednesday, with a young boy killed by a sniper, the Syrian Observatory for Human Rights said.
And battles raged as troops and rebels clashed across the country, with the Observatory saying a total of 44 people were killed on Thursday, 25 of them civilians in Homs province.