Clinton, speaking in Copenhagen, Denmark Thursday, rejected Russia's "vociferous ... claim that they are providing a stabilizing influence" in Syria, saying Moscow was backing President Bashar Assad as his government engaged in a brutal crackdown on pro-democracy dissidents, Voice of America reported.
"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 and China both have trading ties with Syria.
In Washington, White House spokesman Jay Carney warned about the danger of Syria's unrest disintegrating into a situation that draws in Iran and other regional powers.
"The longer Assad and his thugs are allowed to brutally murder the Syrian people," Carney said, "the more likely it becomes a sectarian civil war, the more likely that it spills over Syrian borders."