US Secretary of State Hillary Clinton has urged allies at a Paris meeting on Syria to demand from Russia and China that they end their support for the regime of President Bashar al-Assad.She said Moscow and Beijing had nothing to gain by standing up for Mr Assad, and their actions were intolerable.
More than 100 countries, known as Friends of Syria, are making a fresh attempt to decide what should be done to end the violence in Syria.
Russia and China are not attending.
On Thursday it emerged a senior member of Syria's regime had deserted it and fled to Turkey.
The pro-government website Syriasteps said Brig Gen Manaf Tlas had made an "escape" but that this was "insignificant".
Sources close to the family confirmed his departure to the BBC. But it is not clear whether he has defected to the opposition.
If confirmed it would be the highest-level defection of a supporter of President Assad since the rebellion against him started 16 months ago.
Earlier rumours about his possible defection in March proved to be false.
Brig Gen Tlas, of the Republican Guard, is the son of a former Syrian defence minister. As a young man he attended military training with Mr Assad.
AnalysisSources have confirmed to the BBC that General Tlas has gone to Turkey. But it's not yet clear whether or not his journey counts as a defection. The whereabouts of his immediate family are unclear. Earlier rumours about his possible defection in March proved to be false. So the general's intentions may only become clear if and when he appears in public to outline his plans.
Such detailed attention is paid to his movements because Manaf Tlas is a member of Syria's most powerful Sunni family. For decades, this family has given its support to the Assad family - which is from Syria's Alawite minority. Manaf Tlas's 80-year-old father Mustafa - who is also believed to be abroad - was Syria's defence minister for many years and helped to ensure Bashar al-Assad's succession to the presidency 12 years ago.
If Manaf Tlas's permanent departure from Syria is confirmed, it would mark the first break of a member of President Assad's close circle. Observers believe that Gen Tlas's defection - if confirmed - may encourage other Sunni officers to consider their allegiances.
Mrs Clinton called on the UN Security Council to pass a resolution imposing immediate sanctions on Syria."We should go back and ask for a resolution in the Security Council that imposes real and immediate consequences for non-compliance, including sanctions under Chapter 7 [allowing for the possibility of military force]," she said.( To be next libya )
She said Russia and China had to be persuaded to end their support for the regime.
"I ask you to reach out to Russia and China and to not only urge but demand that they get off the sidelines and begin to support the legitimate aspirations of the Syrian people," she said.
"I don't think Russia and China believe they are paying any price at all, nothing at all, for standing up on behalf of the Assad regime."
The Paris meeting follows similar events in Tunis and Istanbul which demanded tougher action against the Assad regime.
The US and its Western allies are expected to encourage Gulf states to do more to push for global sanctions.
But Russia and China, which both hold vetoes in the Security Council, are not at the meeting.
Opening the meeting, French President Francois Hollande said it was a "human and political necessity" to stop the conflict.
"The crisis has become a threat to international peace and security," he said, "and we have to draw the conclusions."
UN diplomats are working on a document calling for restrictions on commercial activity if Mr Assad fails to abide by UN and Arab League envoy Kofi Annan's ceasefire plan and roadmap for a political transition.
The roadmap - announced last weekend by Annan after a meeting of world powers in Geneva - includes an interim government to enable the Syrian people to live ''independently and democratically''.
The Western powers believe that Mr Assad should play no part in Syria's future, but the roadmap allows Mr Assad an effective veto over any interim candidate he opposes.
Meanwhile UN Secretary General Ban Ki-moon is expected to release a report on the status of the UN monitoring mission in Syria.
The BBC's Barbara Plett at the UN says the general expectation is that Mr Ban will recommend a downscaling of the mission, keeping its civilian component - political and human rights experts - and significantly reducing the military observer part.
Some 15,800 people have died in more than a year of violence in Syria, activists say.