Thirteen countries on Saturday voted for the resolution proposed by European and Arab nations to give strong backing to the Arab League's plan to end the crackdown.
But Russia and China made a repeat of their rare double veto carried out on October 5.
The move was immediately condemned by New York-based Human Rights Watch (HRW) as "diplomatic cover" for the Syrian government.
In a statement, the rights group said: "Vetoes by Russia and China are not only a slap in the face of the Arab League, they are also a betrayal of the Syrian people."
The HRW statement continued: "The death toll had more than doubled in the last four months, and the risk is high that the Assad regime will see this double veto as a green light for even more violence."
Double veto 'unforgivable'
Susan Rice, the US envoy to the UN, called the double veto "unforgivable".
"Since these two members last vetoed the resolution an estimated 3,000 Syrians have been killed, with nearly 250 killed just yesterday. Many thousands more have been held and tortured," Rice said, adding "Once again, the courageous people of Syria can see which members of the council support their fight and which do not."
Rice concluded with a direct warning to Moscow and Beijing that "any further bloodshed will be on their hands".
Moscow, a strong ally of the Syrian government, had earlier signaled it would veto any call for President Bashar al-Assad's removal.
Al Jazeera meets activists in Homs who are defying bullets to document violence
The diplomatic developments come with activists reporting on Saturday that a Syrian army assault on Homs’ neighbourhood had killed more than 200 civilians since Friday night.
The UK-based Syrian Observatory for Human Rights cited witnesses saying 217 people had been killed in Homs, 138 of them in the Khaldiyeh neighbourhood.
The opposition Syrian National Council decried Saturday's violence as a "horrific massacre".
"The Syrian National Council calls on everyone around the world to speak up and do something to stop the bloodshed of innocent Syrians," it said in a statement.
The Local Co-ordination Committees, an activist network, said more than 500 additional people were injured after the army used tanks, mortars and machine guns in the assault on the opposition stronghold.
Al Jazeera's Mysa Khalaf, reporting from Beirut, said sources in Syria told her the bombardment of the area started after the opposition Free Syrian Army, a group of army defectors, attacked Syrian army checkpoints and killed about 10 soldiers.
"I've been told that the main public hospital is completely overwhelmed and people have set up makeshift clinics in mosques. They are running low on supplies of blood," she said. "Several buildings have been destroyed."
The Syrian government denied the assault, saying the reports were part of a 'hysterical campaign'' of incitement by armed groups against Syria.
Embassies attacked
Sergei Lavrov, Russian foreign minister, had said earlier on Saturday that if a resolution is put to a vote without taking Moscow's opinion into account, it will only lead to "another scandal" at the Security Council.
Lavrov said Moscow had objections to what he termed "the imposition of the terms and conditions of the dialogue, which must be started without prejudging the results".
He also said that "measures must be taken to influence not only the government ... but also the armed groups, because unless you do it both ways, you are taking sides in a civil war".
Moscow has been a strategic ally of Syria through its decades under Assad dynastic rule and a major arms supplier to Damascus, and so bristles at outsiders trying to dictate internal political change in Damascus.
Rafeeq Abdel Salaam, Tunisia's foreign minister, announced on Saturday that his government started the procedure for expelling the Syrian ambassador from Tunis.
Earlier on Saturday, British police used batons and riot shields to hold back protesters trying to storm the Syrian embassy in London for the second time in one day.
Police brought in sandbags and riot gear to regain control of the surging crowd, which lobbed objects at the embassy, situated near Buckingham Palace.
Demonstrators angry with Assad stormed five Syrian embassies in Europe and the Middle East, including in Cairo.
The UN has reported that more than 5,400 people have been killed since anti-Assad protests erupted in March 2
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