After Russia and China blocked a UN resolution backing an Arab League plan to urge the Syrian president to give up power on Sunday, US Secretary of State Hillary Clinton called the veto a "travesty". It came a day after activists say Syrian forces bombarded a district of Homs, killing more than 200 people in the worst night of bloodshed of the 11-month uprising.
Russia said the resolution was biased and would have meant taking sides in a civil war. Syria is Moscow's only big ally in the Middle East, home to a Russian naval base and customer for its arms. China's veto appeared to follow Russia's lead.
Washington's UN ambassador Susan Rice said she was "disgusted" by Russia and China's vetoes on Saturday, and "any further bloodshed that flows will be on their hands".
British Foreign Secretary William Hague said Moscow and Beijing had turned their backs on the Arab world. France's Alain Juppe said they "carried a terrible responsibility in the eyes of the world and Syrian people".
Clinton said the United States would work with other nations to try to tighten "regional and national" sanctions against Assad's government "to dry up the sources of funding and the arms shipments that are keeping the regime's war machine going."
"We will work to expose those who are still funding the regime and sending it weapons that are used against defenseless Syrians, including women and children," she said. "We will work with the friends of a democratic Syria around the world to support the opposition's peaceful political plans for change."
Clinton did not give further details which nations might band together or precisely what they might do. But it appeared that the United States might seek to help organise a "Friends of Syria" group - proposed by French President Nicolas Sarkozy after the veto - to advance the Arab League initiative given the inability to make headway at the UN due to Russia and China.
All 13 other members of the Security Council voted to back the resolution, which would have "fully supported" the Arab League plan for Assad to cede powers to a deputy, a withdrawal of troops from towns and a start to a transition to democracy.
The Western criticism was echoed in the Middle East, where Arab powers like Saudi Arabia and non-Arab Turkey have turned decisively against Assad in recent months.
"Unfortunately, yesterday in the UN, the Cold War logic continues," said Turkish Foreign Minister Ahmet Davutoglu. "Russia and China did not vote based on the existing realities but more a reflexive attitude against the West."
Arab League head Nabil Elaraby said the body still intends to build support for its plan. The veto "does not negate that there is clear international support for the resolutions of the Arab League," he said in a statement seen by Reuters.
The Security Council's sole Arab member, Morocco, voiced "great regret and disappointment" at the veto. Ambassador Mohammed Loulichki and said the Arabs had no intention of abandoning their plan.
Burhan Ghalioun, head of the opposition umbrella Syrian National Council, called Moscow and Beijing's veto "a new license to kill from these two capitals for Bashar al-Assad and his criminal regime, which just yesterday killed 300 people".
The SNC said it held Moscow and Beijing "responsible for the escalating acts of killing and genocide".
Protesters stormed the Russian embassy in Libya's capital Tripoli on Sunday, climbing on the roof and tearing down the flag. Men held up a banner saying: "Libyan revolutionaries are ready to fight with their brothers in Syria."
Russia's UN envoy, Vitaly Churkin, accused the resolution's backers of "calling for regime change, pushing the opposition towards power and not stopping their provocations and feeding armed struggle".
"Some influential members of the international community, unfortunately including those sitting around this table, from the very beginning of the Syrian process have been undermining the opportunity for a political settlement," he said. Moscow is sending Foreign Minister Sergei Lavrov to Damascus on Tuesday.
Clinton had met Lavrov before Saturday's vote for what US officials called "vigorous" talks.
"What happened yesterday at the United Nations was a travesty," she said on Sunday. "Those countries that refused to support the Arab League plan bear full responsibility for protecting the brutal machine in Damascus."
She added: "Faced with a neutered Security Council, we have to redouble our efforts outside of the United Nations with those allies and partners who support the Syrian people's right to have a better future."
Syria says it is being targeted by the West and by hostile neighbours providing diplomatic cover for an armed insurgency steered from abroad.
Syrian UN envoy Bashar Ja'afari condemned the resolution and its sponsors, which included Saudi Arabia and seven other Arab states, saying nations "that prevent women from attending a soccer match" had no right to preach democracy to Syria.
He also denied that Syrian forces killed hundreds of civilians in Homs, saying "no sensible person" would launch such an attack the night before the Security Council was set to discuss his country.
State television showed live footage of Assad on Sunday praying with Muslim clerics and listening to Koranic verses in a Damascus mosque to mark the birthday of the Prophet Mohammad.
Residents of Homs's battered Baba Amro district, speaking by telephone, denounced the Russian-Chinese veto, some chanting, "Death, rather than disgrace".
One resident who identified himself as Sufyan said: "Now we will show Assad. We're coming, Damascus. Starting today we will show Assad what an armed gang is." Assad has called his opponents "armed gangs" and "terrorists" steered from abroad.
If activists' accounts are accurate, the bombardment of Homs on Friday night was one of the bloodiest episodes of the Arab Spring uprisings sweeping the region and the deadliest incident in the Syrian conflict.
Syrian activist groups gave varying tolls above 200 killed, saying tanks and artillery blasted the Khalidiya neighbourhood of Homs, a restive city that has become a heartland of resistance to Assad's rule.
Rami Abdullrahman of the Britain-based Syrian Observatory for Human Rights said that as of Sunday he had a list of 159 names of people confirmed killed in that incident, lower than the figure cited by the main political opposition. He said his list was not yet complete.
Damascus denies firing on houses and says images of dead bodies on the Internet were staged. Western governments say they believe the activists' account.
"Yesterday the Syrian government murdered hundreds of Syrian citizens, including women and children, in Homs through shelling and other indiscriminate violence, and Syrian forces continue to prevent hundreds of injured civilians from seeking medical help," US President Barack Obama said before the UN vote.
"Any government that brutalises and massacres its people does not deserve to govern," Obama said in a statement.
There were reports of more violence on Sunday. Activist Omar Shakir, in the Baba Amro district of Homs, said there was new shelling on Sunday afternoon and three people had been killed.
Abdulrahman's group said a total of 19 people had been killed by security forces across the country, and that a total of 21 government troops had died fighting with defectors.
Syria's state news agency SANA reported the funerals of three security personnel, adding to its announced toll of some 2,000 government forces killed during the uprising.
Syria restricts access to the country for journalists, and there was no independent verification of any of the reported death tolls in Syria on Sunday.