More than 100 countries meeting in Paris have called for tougher sanctions to bring about change in Syria.
US Secretary of State Hillary Clinton told the allies they should also demand an end to Russian and Chinese support for President Bashar al-Assad's regime.
She said Moscow and Beijing had nothing to gain by standing up for Mr Assad, and their actions were intolerable.
But Russia has rejected her remarks as "inappropriate" and not part of a broad international deal reached in Geneva.
Neither Russia nor China were attending the Paris summit where the international group, known as Friends of Syria, was making a fresh attempt to decide what should be done to end the violence in Syria.
Earlier meetings in Tunis and Istanbul had also demanded more stringent action.
French President Francois Hollande, who opened the event, said it was a "human and political necessity" to stop the conflict.
Activists in Syria said that at least 25 people were killed on Friday when Syrian forces seized the northern city of Khan Sheikhoun from rebels.
As the Paris meeting progressed, France confirmed that a senior member of Syria's regime, Brig Gen Manaf Tlas, had deserted, delivering a "hard blow" to President Assad.
It is not clear whether the general, the son of a former defence minister, has defected to the opposition. However, the head of the opposition Syrian National Council, Abdel Basset Sayda, told journalists in Paris that he would "seek cooperation with him".
After the meeting, the 107 countries involved issued a communique urging the UN Security Council to adopt urgently UN-Arab League envoy Kofi Annan's six point plan under Chapter 7 of the UN Charter. Russia and China both hold vetoes in the Security Council.
Although Chapter 7 allows for military intervention, they said the measures had to be enforced under Article 41 of the charter which refers to economic, diplomatic, travel and communication measures.
The sanctions working group had recommended that every state freeze senior regime officials' assets, Mrs Clinton told reporters.
Transactions with Syria's commercial and central banks should also be restricted, she said, and there should be an embargo on Syrian oil.
"The entire world is now looking to those few nations that still have influence in Damascus. They need to step up and use all their leverage to make sure Assad sees the writing on the wall. Sitting on the sidelines or, even worse, enabling the regime's brutality would be a grievous mistake."
Although Russia and China were not explicitly named, Mrs Clinton had spoken earlier of both countries as remaining on the sidelines.
"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."
But Russian Deputy Foreign Minister Gennady Gatilov said the US secretary of state's remarks were "inappropriate" and worrying.
He told Interfax news agency that Mrs Clinton had appeared to contradict what had been agreed after a meeting of world powers in Geneva last weekend.
Moscow and the US were among the countries backing Kofi Annan's roadmap for political transition, which 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.