NBC's Ayman Mohyeldin reports.
Updated at 4:09 p.m. ET: Amid escalating bloodshed in the Syrian city of Homs, the U.N. Security Council on Saturday failed to pass a resolution calling on the Syrian president to step down.
Russia and China vetoed the resolution endorsing an Arab League call for Bashar Assad to leave power. The other 13 council members, including the U.S., France and Britain, voted in favor of the resolution.
The vote took place as Syrian forces pummeled the city of Homs with mortar and artillery fire that activists say killed more than 200 people in one of the bloodiest episodes of the uprising against Assad's regime. The U.N. says more than 5,400 people have been killed over almost 11 months in a Syrian government crackdown on civilian protests.
NBC's Andrea Mitchell reports.
"The United States is disgusted that a couple of members of this Council continue to prevent us from fulfilling our sole purpose here -- addressing an ever-deepening crisis in Syria and a growing threat to regional peace and security," said Susan Rice, the U.S. ambassador to the U.N.
"This is a great disappointment to the people of Syria and the Middle East, and to all supporters of democracy and human rights," a spokesperson for U.N. Secretary-General Ban Ki-moon said. "It undermines the role of the United Nations and the international community in this period when the Syrian authorities must hear a unified voice calling for an immediate end to its violence against the Syrian people."
NBC's Richard Engel and Ali Arouzi report on the escalating tension between the two nations.
U.S. Secretary of State Hillary Clinton said it had not been possible to work constructively with Russia ahead of the vote. "I thought that there might be some ways to bridge, even at this last moment, a few of the concerns that the Russians had. I offered to work in a constructive manner to do so. That has not been possible,'' she told reporters at a security conference in Munich.
Syria has been a key Russian ally since Soviet times and Moscow has opposed any U.N. call that could be interpreted as advocating military intervention or regime change.
Russian Ambassador Vitaly Churkin told reporters that he was encouraged by statements about "the intention to continue diplomatic efforts" and noted that the Security Council is "not the only diplomatic tool on the planet."
Churkin said that Russian Foreign Minister Sergey Lavrov and the country's foreign intelligence chief, Mikhail Fradkov, will meet with Assad in Damascus on Tuesday, without providing specifics on the purpose of that trip.
Russia had expressed concerns about the draft text, saying it feared the resolution would lead to the kind of military intervention and regime change seen in Libya after last year's council action intended to protect civilians from attacks by forces loyal to strongman Moammar Gadhafi.
A view of houses that residents said were damaged during a military crackdown on protesters against Syrian President Bashar al-Assad, in Rasten near Homs.
After Saturday's vote, Churkin accused fellow council members of being inflexible.
"We greatly regret this result of our joint work" on the resolution, he said.
Earlier, President Barack Obama issued a statement calling the latest attack on Homs "an unspeakable assault" by Assad's regime.
"Assad must halt his campaign of killing and crimes against his own people now. He must step aside and allow a democratic transition to proceed immediately," Obama said.
"The Syrian regime's policy of maintaining power by terrorizing its people only indicates its inherent weakness and inevitable collapse," Obama said. "Assad has no right to lead Syria, and has lost all legitimacy with his people and the internationalcommunity."
The violence in Homs sparked protests in several cities outside Syria.
British police used batons and riot shields to try to hold back protesters trying to storm the Syrian Embassy in London. "We want to close the embassy!" demonstrators shouted.
Police brought in sandbags and riot gear to regain control of the surging crowd, which lobbed objects at the embassy, situated near Buckingham Palace.
In all, anti-Assad demonstrators stormed at least five embassies in Europe and the Middle East.
The Asociated Press and Reuters contributed to this story.
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