By Michelle Nichols
UNITED NATIONS, July 19 (Reuters) - Russia and China vetoed a U.N. Security Council resolution on Thursday that threatened Syrian authorities with sanctions if they did not halt violence against an uprising, thwarting Western hopes for tough action as the crisis in Syria escalates.
It was the third time that Russia, a key ally of the Syrian government, and China have used their veto power to block U.N. Security Council resolutions designed to put pressure on Syrian President Bashar al-Assad and halt the violence in a 16-month conflict that has killed thousands of people.
As efforts to forge a diplomatic solution appeared to collapse, Syrian rebels clashed with forces loyal to Assad in Damascus, a day after the killing of three close allies of the president led to fresh warnings that the situation was spinning out of control.
The vetoed resolution, which would have extended a U.N. observer mission in Syria for 45 days, received 11 votes in favor, while South Africa and Pakistan abstained.
International mediator Kofi Annan, who had sought a tough resolution to save his disintegrating peace plan, voiced disappointment, saying the council had failed "to take the strong and concerted action he had urged and hoped for."
Russia, which had proposed its own alternative resolution without the sanctions threat, said it did not now plan to bring that measure to a vote, leaving the future of the monitoring mission hanging in the balance less than two days before its U.N. mandate expires.
Britain proposed a resolution to extend the monitoring mission for 30 days and hoped to put it to a vote on Thursday, diplomats said, but it remained unclear how much backing this new measure might win.
The U.S. ambassador to the United Nations, Susan Rice, described the twin vetoes as "dangerous and deplorable" and said the Security Council had "failed utterly."
"The United States has not, and will not, pin its policy on an unarmed observer mission that is deployed in the midst of such widespread violence and that cannot even count on the most minimal support of this Security Council," Rice said.
"Instead, we will intensify our work with a diverse range of partners outside the Security Council to bring pressure to bear on the Assad regime and to deliver assistance to those in need."
The United States has been instrumental in forming the "Friends of Syria" group, which includes a wide range of Western and Arab countries, that has sought to increase pressure on Damascus and encourage more unity among Syria's fractured opposition.
Rice noted that the escalating violence was sharpening concerns over Syria's undeclared stockpile of chemical weapons, believed to be the largest of its kind in the Middle East.
"As the situation deteriorates, the potential that this regime could consider using chemical weapons against its own people should be a concern for us all," she said.
The British and French U.N. envoys also expressed their disgust over Russia and China's vetoes.
"The effect of their actions is to protect a brutal regime. They have chosen to put their national interests ahead of the lives of millions of Syrians," British Ambassador Mark Lyall Grant told the council.
MANDATE ALMOST UP
The 15-member council still has time to negotiate another resolution on the fate of the unarmed U.N. observer mission before its initial 90-day mandate expires at midnight on Friday (0400 GMT Saturday).
Russia, China, Pakistan and South Africa have called for a brief technical resolution that would simply roll-over the mission for a short period of time.
"It would be useful to preserve the useful potential of the monitoring mission, even if it is limited in its time," Russia's U.N. envoy, Vitaly Churkin, said.
Lyall Grant said Britain's new resolution would only consider extending the mission beyond an additional 30 days if the Syrian authorities had stopped using heavy weapons and had withdrawn troops from towns and cities - and if the level of violence had decreased enough to allow the mission to work.
"We hope it may be possible to vote that later in the day, he told reporters.
Since the Security Council approved the initial deployment of the mission, the United States has indicated that Washington is unlikely to support a renewal of their mandate if the Syrian government had not implemented Annan's six-point peace plan.
White House spokesman Jay Carney, briefing reporters aboard Air Force One as President Barack Obama headed for Florida, said the United States does not support extending the monitor mission after the failed resolution.
But Rice, the U.N. ambassador, indicated that Washington might consider a "final brief extension."
Britain, France, Germany and the United States had wanted the Security Council resolution be placed under Chapter 7 of the U.N. Charter, which allows the council to authorize actions ranging from diplomatic and economic sanctions to military intervention.
Western council members have said they are talking about a threat of sanctions on Syria, not military intervention. Their vetoed resolution had contained a specific threat of sanctions if Syrian authorities did not stop using heavy weapons and withdraw troops from towns and cities within 10 days.
Russia made clear days before the vote it would block any such move, with Russian Foreign Minister Sergei Lavrov describing the threat of sanctions as "blackmail."
The Security Council initially approved the deployment of the observer mission, known as UNSMIS, to monitor a failed April 12 ceasefire. UNSMIS suspended most of its monitoring activity on June 16 due to increased risk from rising violence. (Additional reporting by Matt Spetalnick in Washington and Stephanie Nebehay in Geneva; Writing by Andrew Quinn; Editing by David Brunnstrom)