b>Washington: The US has blamed Kofi Annan's resignation as special UN envoy to Syria on the failure of Russia and China to back his peace plan for the end of 17-month conflict in that country.

"Annan's resignation highlights the failure at the United Nations Security Council of Russia and China to support meaningful resolutions against Assad that would have held Assad accountable for his failure to abide by his commitments under the Annan plan," White House Press Secretary Jay Carney said.

"Those vetoes, as we've said repeatedly, were highly regrettable, and place both Russia and China on the wrong side of history and on the wrong side of the Syrian people," he said.

The Secretary of State, Hillary Clinton, later spoke to Annan over phone.

"Unfortunately, the Security Council was blocked from giving him key tools to advance his efforts," Clinton said in a statement.

Annan, she said, worked tirelessly to try to build consensus in the international community, end the bloodshed, and usher in a government that would meet the legitimate aspirations of the Syrian people.

"To the Syrian people: the United States continues to stand with you and we remain committed to an effective and swift political transition as envisioned under the Annan framework," Clinton said.

Earlier, Carney said Assad, despite his promise to abide by Annan's plan, continues to brutally murder his own people, to use heavy weapons in assaults on civilian population centers, to call on his military leaders to kill the Syrian people in his name.

"It is disgusting and only highlights the absolute requirement that, for the future of the Syrian people, Assad must step aside," he said.