China and Russia have vetoed a UN Security Council resolution condemning Syria over its crackdown on anti-government protesters.
The European-drafted resolution had been watered down to try to avoid the vetoes, dropping a direct reference to sanctions against Damascus.
But Moscow and Beijing said the draft contained no provision against outside military intervention in Syria.
The US envoy to the UN said Washington was "outraged" by the vote.
Susan Rice, who walked out after the vote, said opposition to the resolution was a "cheap ruse by those who would rather sell arms to the Syrian regime than stand with the Syrian people".
The result is a huge blow to European and US efforts on the Syria issue, the BBC's Laura Trevelyan in New York says.
More than 2,700 people have been killed across Syria since the crackdown began in March, the UN estimates.
The government of Syrian President Bashar al-Assad says it is in the process of introducing reforms and is speaking to members of the opposition. It blames the unrest on armed gangs.
Nine countries backed the resolution in the 15-member council, while four more abstained during the vote late on Tuesday.
But the resolution - which was drafted by France with the co-operation of Britain, Germany and Portugal - was still defeated because of the vetoes from two of the council's five permanent members.
This was despite the fact that the text had been changed three times by the European allies, who had tried to accommodate Russia's and China's objections.
The resolution referred to "targeted measures" - instead of sanctions - if the clampdown in Syria continued.
But Russia's UN Ambassador Vitaly Churkin said that draft was based on "the philosophy of confrontation".
He said the threat of an ultimatum of sanctions against the Syrian authorities was "unacceptable" and that the approach went "against the principle of a peaceful settlement of a crisis on the basis of a full Syrian national dialogue".
He repeated Moscow's concerns that a resolution could lead to a Libyan-style foreign military intervention in Syria and said pressure should also be put on the Syrian opposition movement to refrain from violence and disassociate itself from "extremists".
But Mr Churkin added that Moscow would like the Syrian regime to be "quicker with implementing the promised changes".
China's UN ambassador Li Baodong said that Beijing opposed the idea of "interference in (Syria's) internal affairs."
He added that "sanction or threat of sanction does not help resolve the question of Syria" but "may further complicate the situation".
The Libyan conflict has sharpened divisions among Security Council members, with both Moscow and Beijing saying that the resolution authorising the use of force to protect civilians was misused by Nato to bring down Col Muammar Gaddafi's regime.
Our correspondent says Tuesday's vote exposes the deep rift at the UN between the major powers, and the complete difference of approach in how to deal with Mr Assad's government and its crackdown.
After the vote, France's UN Ambassador Gerard Araud said the veto showed "disdain for the legitimate interests that have been fought for in Syria" since the protests in the country began.
But he added that "no veto can give give carte blanche to the Syrian parties who have lost full legitimacy in assassinating and killing their people".
"Calls from the Arab League for an end to this bloodbath and statements from neighbouring countries to the suffering of the Syrian people show that this veto goes against the sense of history underway in Syria and throughout the region," said Mr Araud.
The US envoy, Susan Rice, said: "The United States is outraged that this council has utterly failed to address an urgent moral challenge and a growing threat to regional peace and security.
"Today the courageous people of Syria can now see who on this council supports their yearning for liberty and universal human rights and who does not," she said.
Syria's envoy to the UN, Bashar Jaafari, said his country was "astounded" by the council's efforts to "undermine stability, security and co-existence" in the region and in Syria.
Ms Rice walked out in protest when Mr Bashar said the US was using its veto power to protect Israel, which could be seen as "partaking in a genocide".
Washington said earlier it hoped the council would send a strong message to Syria. Germany said the world had to show solidarity with Syrians on the streets.