Diplomatic efforts at the United Nations Security Council to address the Syria crisis suffered a potentially fatal blow on Thursday when Russia and China vetoed a British-sponsored resolution that would have punished the Syrian government with economic sanctions for failing to carry out a peace plan.

It was the third time that Russia and China had vetoed resolutions on Syria since the uprising against President Bashar al-Assad began, and the collegial atmosphere in the Council chambers was tinged with bitterness and acrimony afterward.

The British ambassador, Sir Mark Lyall Grant, told the Council after the vote, which was broadcast live on the United Nations’ Web site, that Britain was “appalled by the decision of Russia and China to veto this resolution aimed at ending the bloodshed in Syria.”

Speaking to reporters later outside the Council’s chambers, Sir Mark said, “Frankly, it is impossible to understand why China and Russia felt it necessary to veto this text.”