US Secretary of State Hillary Clinton has urged allies at a meeting on Syria to demand from Russia and China that they end their support for the regime of President Bashar al-Assad.
Representatives from more than 100 countries, known as Friends of Syria, are making a fresh attempt to decide what should be done to end the violence which began in March last year.
Mrs Clinton told the meeting in Paris on Friday that Moscow and Beijing had nothing to gain by standing up for Mr Assad and their actions were intolerable, the BBC reports.
She called on the United Nations 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.
Mrs Clinton 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.
"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, France's president Francois Hollande said Syria's crisis is now a threat to world peace and security and it was a "human and political necessity" to stop the conflict.
General deserts regime
A Syrian general from a powerful family close to President Bashar al-Assad has deserted the regime.
Sources close to the family of Brigadier-General Manaf Tlas say he is heading to Paris via Turkey.
If confirmed, it would be the highest-level defection since the unrest began in Syria 16 months ago, the BBC reports.
Sources say General Tlas has been under a form of home arrest since May last year because he opposed the security solution the regime has been implementing.
Unlike most of Syria's Alawite leaders, he is a Sunni Muslim; but for decades his family has given support to the Assad family.