Britain and the US are furious with Russia and China for vetoing a draft UN Security Council resolution to impose sanctions on Zimbabwe's leaders.
UK Foreign Secretary David Miliband said their veto was incomprehensible, especially as Russia had earlier suggested it backed tougher action.
Russia had given an understanding to the UK and the US at the latest G8 summit that they would support the UN sanctions vote.
China were always going to be harder to convince as they supply the Zimbabwe regime with arms.
Zimbabwe's UN ambassador said the UK and US had come up with flimsy reasons.
Boniface Chidyausiku said he was happy to see what he called the machinations of the two failing.
Chinese Ambassador Wang Guangya said his nation believes the international community should allow the talks in South Africa on the Zimbabwe situation a chance to progress before imposing sanctions.
Wang said the adoption of the resolution would "unavoidably interfere with the negotiation process and lead to the further deterioration of the situation."
"Many countries, including China, repeatedly called upon the Security Council to respect the position of the African countries on this question and give more time," Wang said.
"China has always maintained the best approach to solve a problem is negotiation and dialogue," Wang said. "To use or threaten to use sanctions lightly is not conducive to solving the problem."
U.S. Ambassador Zalmay Khalilzad said the United States was disappointed at the veto.
"China and Russia have stood with Mugabe against the people of Zimbabwe ... for reasons that we think are not borne out by the facts on the ground," he said.
"The U-turn on the Russian position is particularly surprising and disturbing," he said, noting that a few days ago, the Russian Federation was supportive of a statement adopted at the Group of Eight meeting in Japan expressing "grave concern" about the situation in Zimbabwe.
"The Russian performance here today raises questions about its reliability as a G-8 partner," Khalilzad said.
