(Reuters) - Russia and China vetoed on Saturday a U.N. resolution that backed an Arab plan calling on Syrian President Bashar al-Assad to quit, stalling global efforts to end his bloody crackdown on unrest after hundreds were reported killed in the city of Homs.

The high-level diplomatic setback came after world leaders and Syrian opposition activists accused Assad's forces of a massacre in a sustained shelling of Homs, the bloodiest episode in 11 months of upheaval in the pivotal Arab country.

Russia and China joined in a double veto of a Western- and Arab-driven resolution at the U.N. Security Council endorsing the Arab League plan for Assad to hand power to a deputy to make way for a transition towards democracy.

The other 13 council members voted for the resolution that would have said the council "fully supports" the League plan aimed at stopping Syria's bloodshed, whose sectarian overtones threaten stability in the wider Middle East region.

Russia complained that the draft resolution was an improper and biased attempt at "regime change" in Syria, which is Moscow's sole major Middle East ally, an important buyer of Russian arms exports and host to a Russian naval base.

With an eye to events in Homs, U.S. Ambassador to the U.N. Susan Rice dispensed with the usual diplomatic courtesies and declared she was "disgusted" by the Russian-Chinese veto, adding that "any further bloodshed that flows will be on their hands".

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