Clinton said in a statement Friday that Syrian President Bashar Assad's regime is deliberately murdering innocent civilians as it tries to put down 16 months of unrest and that history will judge the Security Council if it fails to act.
"History will judge this council," she said. "Its members must ask themselves whether continuing to allow the Assad regime to commit unspeakable violence against its own people is the legacy they want to leave."
Although she did not mention them by name, Clinton's warning was directed at Russia and China, which have blocked previous U.N. efforts to impose sanctions on Syria.
The U.S. and its allies want the council to give full endorsement to a plan for political transition in Syria put forward by U.N. envoy Kofi Annan and lay out consequences for noncompliance with the proposal.
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Earlier, U.N. chief Ban Ki-moon accused the Syrian government of violating U.N. resolutions by using heavy weapons against the village of Tremseh, where reports say as many as 100 were killed and urged the council to take "collective action" in response.
The opposition says the attack on Tremseh - in which gunners rained shells on the village before armed thugs moved in - marked one of the worst single days of bloodshed in the uprising against Assad's regime.