US ambassador to the UN Susan Rice said she was ‘disgusted’ by the Russian and Chinese veto on a UN Security Council resolution on Syria that urges President Bashar al-Assad to step down.
Thirteen of the council's 15 members voted in favour of the resolution aimed to stop the ongoing violence in Syria.
"Any further bloodshed that flows will be on their (Russia's and China's) hands," she said.
US Secretary of State Hillary Clinton said the failure to condemn al-Assad increased the risk of more bloodshed and civil war in Syria.
"If we do not begin the process, I know what will happen: more bloodshed, increasing resistance by those whose families are being killed and whose homes are being bombed, and a greater likelihood that Syria will descend into civil war," Clinton said.
Russian Ambassador Vitaly Churkin said the draft resolution "did not reflect Syria's realities well enough and sent conflicting signals to the political forces in Syria".
Russian Foreign Minister Sergei Lavrov earlier said the resolution did not set enough demands on anti-government armed groups, and that Russia was concerned it could jeopardize the national dialogue among political forces in Syria.
Lavrov and Foreign Intelligence Service head Mikhail Fradkov will visit Syria and meet al-Assad on February 7.
This was the second time that Russia and China as permanent members have vetoed the Syria issue. In October, they blocked a European-sponsored resolution condemning Syria and threatening possible sanctions.
At least 5,400 people have been killed in the Syrian government's 11-month crackdown on protesters, according to the UN. Syrian authorities blame the violence on armed gangs affiliated with Al Qaeda and say more than 2,000 soldiers and police have been killed.