Speaking in Denmark Thursday, Clinton said she rejects the Russians' "vociferous...claim that they are providing a stabilizing influence" in Syria. Instead, she said, Moscow is propping up Mr. Assad as his government continues a brutal crackdown on dissent U.N. estimates say has killed more than 10,000 people.
"We have very strong opposition from Russia and China, but it is primarily Russia," Clinton said, "and that makes it harder to put together an international coalition."
Russia has repeatedly blocked the U.N. Security Council from taking punitive action against the Assad government, a longtime Russian ally.
White House spokesman Jay Carney echoed Clinton's warnings about the danger of Syria's unrest morphing into a proxy war that draws in Iran and other regional powers.
"The longer Assad and his thugs are allowed to brutally murder the Syrian people," he said, "the more likely it becomes a sectarian civil war, the more likely that it spills over Syrian borders.'' Carney warned of a "proxy war" with Iran backing Mr. Assad and other outside nations or forces backing insurgent factions.
Also Thursday, U.S. ambassador to the United Nations Susan Rice condemned as "reprehensible" the latest reported Russian arms delivery to Syria. Media reports and rights groups said a Russian cargo ship heavily laden with weapons docked at the Syrian port of Tartus last week.
Western officials confirmed her comments, adding they understood the ship had been carrying arms for the Syrian government.
Syria is one of Russia's top weapons customers. The U.S. and European Union have suggested the U.N. Security Council should impose an arms embargo and other international sanctions on Syria.
U.S. and European security officials say Iran has also offered Assad extensive support to help him suppress anti-government protests, from high-tech surveillance technology to guns and ammunition.
Last week's massacre of at least 108 people in the Syrian town of Houla - nearly half of them children - has rekindled new urgency into efforts to stem the 15-month-old conflict. The U.N. has said much of the killing was carried out at close range, while activists say pro-government militia known as shabiha were responsible.
Syria Government Explanation
The Syrian government said Thursday a preliminary investigation into the atrocity revealed that hundreds of armed men had attacked families who did not join anti-government protests. Brigadier General Qassem Suleiman said the victims were families that "refused to rise up against the government."
Rice called the Syrian report "another blatant lie."
The head of the Britain-based Syrian Observatory for Human Rights said Thursday that government troops killed three people in overnight shelling near Houla. Rami Abdel Rahman said a 14-year-old boy was later killed by sniper fire.
Damascus-based U.N. spokeswoman Sausan Ghosheh said U.N. monitors based in nearby Homs were traveling to Houla to verify the reports of renewed attacks.
Rebel Deadline
In a statement Thursday, a rebel Free Syrian Army commander gave Assad a deadline of noon Friday local time to start acting on commitments made to international peace envoy Kofi Annan.
Annan was discussing the Syrian crisis with Lebanese officials in Beirut Thursday after talks with Jordanian King Abdullah in Amman.
Rebel Colonel Qassim Saadeddine said his forces would no longer be bound by the Annan peace plan if the Syrian president fails to comply.
The Free Syrian Army is a loosely-organized and lightly-armed rebel group made largely of Syrian military defectors. The Syrian government and the rebels agreed in April to a truce mediated by Annan, but the fighting has continued, with each side accusing the other of violating the deal.
Analyst James Denselow of King's College in London says it would be a mistake for the Free Syrian Army to cut itself off from the U.N.-brokered cease-fire.
"I think they would be falling into a trap if they were to be the ones who unilaterally declared the cease-fire over, because that would allow Assad and the regime to say 'well, they broke it, we still believe in it,' despite the fact that they haven't really been properly observing it," Denselow said.
Cherif Bassiouni of DePaul University said Mr. Assad's government will not in the long run be shielded from prosecution.
"They should know that even though Russia and China are protecting them tomorrow, that if there is a commission that investigates what they're doing and has the evidence, they will not always be immune from prosecution in the future. And this has to be done now while the evidence is fresh," Bassiouni said.
U.N. Secretary-General Ban Ki-moon said Thursday he "demands" the Syrian government abide by its peace pledges. Speaking at a forum in Istanbul, Mr. Ban said the almost 300-member U.N. military observer team in Syria is not meant to play the role of "passive observer to unspeakable atrocities."
The U.N. chief also warned that more massacres such as the Houla incident "could plunge Syria into a catastrophic civil war ... from which the country would never recover."
