Growing demands for tough UN Security Council action on Syria are putting increasing pressure on Russia's diplomatic shield around President Bashar al-Assad.
After Assad brushed aside a Tuesday deadline to withdraw troops and guns from cities, the prospects for international pressure will depend on what UN-Arab League envoy Kofi Annan requests, and how far Syria has strained the patience of Russia, its main ally, diplomats say.
Annan is now insisting the Syrian leader make a "fundamental change" and keep a Thursday zero hour for a complete halt to hostilities. But the United States, France, Britain and Turkey say Assad has already failed and are demanding sanctions-style action.
Susan Rice, US ambassador to the UN, warned that the UN Security Council faces a looming "moment of truth" on Syria.
The "logical next step" would be to increase pressure through "collective action," she said.
France's Foreign Minister Alain Juppe said he wants "new measures" by the council after Thursday. Britain's Foreign Secretary William Hague said the Security Council should refer Assad to the International Criminal Court. Turkey, struggling with a wave of Syrian refugees, has also demanded action.
Russia and China vetoed two resolutions on Syria because they hinted at sanctions. They have however signed up to less weighty statements which backed Annan's six-point peace plan and his deadlines to Assad.
Annan has been praised for the way he has built up international unity behind his efforts. One step off the diplomatic tightrope could unleash new Security Council hostilities on Syria, diplomats said.
Rice said Russia and China have a special "responsibility to use the influence they have to end the killing of the Assad regime" -- more than 9,000 dead according to the UN.
For the moment, Annan is "off-limits" to Russian attacks, said one senior UN envoy. "But if he recommends sanctions, the Russians and Chinese could easily turn on him."
Russia's Foreign Minister Sergei Lavrov complained publicly on Tuesday to Syrian counterpart Walid Muallem that the regime should be more "decisive" in carrying out the Annan plan.
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But he also said Annan should put more pressure on the Syrian opposition.
"At best there is uncertainty now about the Russian attitude," said another UN diplomat.
Some analysts say Annan's plan gave leeway to Assad because he had to weaken it to get Russian agreement.
"It was clear that the regime would use what wriggle room existed, and the Annan mission built consensus around a conceptual roadmap that provided plenty of it," said Peter Harling, the International Crisis Group think tank's Middle East specialist.
Annan's plan was "a non-starter" because it contained no talk of Syria's president stepping aside, said Murhaf Jouejati, a professor at the US National Defense University and a member of the opposition Syrian National Council.
Others say however that Annan has skillfully managed his showdown with Assad by keeping the support of Russia and China.
"In a very methodical and moderate way he has revealed the extent of the Assad regime's duplicity," said Richard Gowan, of the New York University (NYU) Center on International Cooperation, who closely follows Security Council affairs.
"The hope was that because Annan put forward a fairly moderate plan that Russia would really put pressure on Syria to accept it," he said.
"Either Russia does not have the influence or it did not put full pressure on to get Assad to
comply. Either way it looks pretty bad for Moscow."
Jouejati predicted that neither Russia nor China would now veto any resolution on economic sanctions against Assad, though nothing military.
But Gowan said it was over-optimistic to think that they would be "shamed" into allowing sanctions.
"Russia's efforts to persuade Syria have been genuine but when it comes to the Security Council, they will continue to block any serious action by the council," he said.
US envoy Rice said however that deadlock risks pushing Syria "into full-scale civil war -- with all of the consequences that that entails for the people of Syria, for neighboring countries, and the wider international community."