China and Russia vetoed Thursday a draft UN Security Council resolution to refer Syria to the International Criminal Court for crimes committed by both sides in the three-year civil war.
Western powers pressed for the resolution in the face of mounting atrocities in Syria, including chemical attacks, systematic torture, barrel bombings and blocked aid access.
It was the fourth time China and Russia have blocked Western resolutions on the conflict, paralyzing Security Council efforts to end a war estimated to have killed more than 160,000 people.
The 13 other members of the Security Council voted in favor and Western powers rounded on China and Russia for protecting not just the Syrian regime but also opposition "terrorist groups."
"Our grandchildren will ask us years from now how we could have failed to bring justice to people living in hell on earth," said US ambassador Samantha Power.
"The victims of the Assad regime's industrial killing machine and the victims of terrorist attacks deserve more than to have more dead counted," she added.
Moscow is Syrian President Bashar Assad's closest ally, and has provided him with diplomatic cover throughout the crisis.
Beijing generally aligns with the Russian position.
"It is disgraceful that they have yet again vetoed the Security Council's efforts to take action on human rights violations in Syria," said British ambassador Mark Lyall Grant.
The text, drawn up by France, was co-sponsored by 60 countries, including members of the European Union, Japan, South Korea and several African states.
Syria is not a signatory to the International Criminal Court so only the Security Council can decide whether to refer war crimes or crimes against humanity on its territory to the court.
It did the same for Darfur in 2005 and Libya in 2011.
French ambassador Gerard Araud said before the expected veto it "would be an insult to millions of suffering Syrians."
Western powers said they would continue to document atrocities and press for justice.
Writing in The Wall Street Journal on Thursday, French Foreign Minister Laurent Fabius said the Security Council "must unite."
"Holding the perpetrators of war crimes and crimes against humanity accountable for their actions is a way of obtaining justice for the victims.
"It is also a deterrent to those who continue to commit such actions. Sooner or later, they will be judged," he wrote.
Jan Eliasson, deputy secretary general of the United Nations, told the Security Council before the vote that the chamber's more than three years of disagreement was deeply damaging.
"If members of the Council continue to be unable to agree on a measures that could provide some accountability for the ongoing crimes, the credibility of this body and of the entire organization will continue to suffer," he said before the vote.
But the Russian veto had never been in doubt.
Russian ambassador Vitaly Churkin on Wednesday dismissed the French initiative as a "publicity stunt" that would undermine efforts to find a political solution to the conflict in Syria.
Peace talks have been locked in stalemate since February and international mediator Lakhdar Brahimi has resigned.
Western diplomats say China is embarrassed but was reluctant to oppose Russia again after abstaining from a resolution denouncing the separatist referendum in Crimea in March.
London-based rights group Amnesty International accused China and Russia of displaying "chilling disregard" for the countless victims in Syria.
"A crucial opportunity for justice has been squandered. Once again, Russia and China have abandoned the Syrian people for the sake of salvaging political alliances," it said.
"Not only does this move risk emboldening those who are committing crimes with impunity but it is yet another sign of how the international community is failing Syrians."