"Frankly we think that it is past time for the UN Security Council to speak up," State Department spokeswoman Victoria Nuland told reporters, denouncing Security Council silence on the Syrian regime as "unconscionable."
"And we are again calling on our partners on the Security Council to be willing to take action and speak out for the innocents in Syria who are suffering at the hands of the regime, including Russia," Nuland said.
Russia is chairing the 15-member UN Security Council this month.
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Russia and China in October used a rare double veto to block a Western-backed resolution condemning Assad's regime. Brazil, India, Lebanon and South Africa abstained.
Moscow said then that the action was one-sided and on Tuesday argued that "armed extremist groups" were becoming increasingly reckless as Western pressure on Assad grew.
When asked to comment on Lavrov's remarks, Nuland replied: "We think it's the Assad regime that is immoral in the violence it's perpetrating on its own people."
Nuland said that there have been "some instances" where the opposition has carried out armed attacks against the regime, but the opposition movement has "been peaceful" in general.
But she said Russia should support Arab League calls to admit monitors and a free press if it wants to have any chance of supporting its claims of violence being perpetrated on both sides.
"That's the best way to assess what's really going on and to ensure a balanced picture that the Russians themselves claim is needed," Nuland said.
Nuland suggested that Russia and China were increasingly isolated in their stance on Syria.
"The chorus of countries that are appealing to Russia, to China, to some of the other countries on the Security Council that were reluctant before is growing," she said.
Earlier, Syrian activists said security forces fired on a funeral procession in a restive northwestern border region, killing two people and raising Tuesday's death toll to at least 28.
AP contributed to the report
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