Russia and China have backed a UN resolution that would have imposed sanctions on Syrian individuals, organisations and companies allegedly involved in chemical weapons attacks during the country's six-year civil war.
It is the seventh time Russia has used its veto to protect the Syrian regime from United Nations Security Council action.
Russia had said the vote on the resolution, drafted by France, Britain and the United States, would harm UN-led peace talks between the warring Syrian parties in Geneva, which began last week.
Russian President Vladimir Putin described the draft resolution as "totally inappropriate."
"For my friends in Russia, this resolution is very appropriate," US Ambassador to the UN Nikki Haley told the council after the vote.
"It is a sad day on the Security Council when members start making excuses for other member states killing their own people. The world is definitely a more dangerous place," she said.
The Western-backed resolution followed a joint investigation by the UN and the international chemical weapons watchdog that concluded the Syrian government was behind at least three attacks involving chlorine gas and Isis was responsible for at least one involving mustard gas.
British U.N. Ambassador Matthew Rycroft told the council before the vote: "This is about taking a stand when children are poisoned. It's that simple. It's about taking a stand when civilians are maimed and murdered with toxic weapons."
The use of chlorine gas as a weapon is banned under the Chemical Weapons Convention, which Syria joined in 2013. If inhaled, chlorine gas turns to hydrochloric acid in the lungs and can kill as it burns the lungs and victims drown in their own body fluids.
Syrian President Bashar al-Assad's government has denied its forces have used chemical weapons.
French UN Ambassador Francois Delattre said the failure by the council to act would "send a message of impunity."
China's UN Ambassador Liu Jieyi said it was too early to act because the international investigation was still ongoing.
"We oppose the use of chemical weapons," he added.
The defeated resolution would also have banned all countries from supplying Syria's government with helicopters, which investigators have determined were used in chemical attacks.
It also proposed targeted sanctions of a travel ban and asset freeze on 11 Syrian military commanders and officials, as well as 10 government and related entities.
The resolution got the minimum nine votes in favour. In addition to China and Russia, Bolivia voted No. Three countries abstained: Egypt, Ethiopia and Kazakhstan.
A resolution needs nine votes in favour and no vetoes by the United States, France, Russia, Britain or China to be adopted.
Sherine Tadros, head of the UN office in New York for Amnesty International, said: “By vetoing this resolution Russia and China have displayed a callous disregard for the lives of millions of Syrians.
"Both states are parties to the Chemical Weapons Convention — there is simply no excuse for their vetoes today.
“For six years Russia, with the support of China, has blocked Security Council decisions that would have punitive consequences for the Syrian government.
"This behaviour prevents justice and emboldens all parties to the conflict in Syria to act with indifference to international law. The message coming from the international community is that when it comes to Syria, there are no red lines.”
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