One of President Bashar al-Assad's personal friends has defected and was headed for exile in France on Friday, as the Syrian crisis took on a Cold War tone when Washington threatened to make Russia and  China "pay" for backing the government in Damascus.

Manaf Tlas, a cadet college classmate, Republican Guard general and son of Assad's father's defense chief, has yet to surface abroad, or clearly to throw his lot in with the rebels, who acknowledged the loss of one of their strongholds overnight.

But his desertion, leaked by family friends, was confirmed by the French government. That gave a boost to a conference it hosted in Paris at which Western powers and Sunni Arab rulers, bitterly opposed to Assad's Iranian-sponsored administration, agreed to "massively increase" aid to the Syrian opposition.

The departure of Tlas, a glamorous, 40-something fixture of both Damascus society and Assad's praetorian close protection force, may have limited practical impact. But it offers succour to an opposition that is divided and poorly armed, as well as to foreign backers whose assurances of flagging morale in Assad's inner circle had begun to wear thin after 16 months of conflict.

"If people like him, and like the generals and colonels and others who have recently defected to Turkey are any indication, regime insiders and the military establishment are starting to vote with their feet," U.S. Secretary of State Hillary Clinton said in Paris after a speech lambasting Moscow and Beijing in unusually strong terms for blocking U.N. action against Assad.

"The only way that will change is if every nation represented here directly and urgently makes it clear that Russia and China will pay a price because they are holding up progress, blockading it," she told the 50 or so delegations.

However, as the past year has shown, options for Assad's foes to pressure his defenders remain limited, given the economic and political strength of both Russia and China.

Moscow, an ally since the Cold War days of Assad's father, has supported a new proposal by U.N. envoy Kofi Annan for a transitional, national unity government in Damascus. But, like China, Moscow is wary of what it sees as a growing Western taste for "regime change", and opposes forcing Assad to step down.

In response to Clinton's remarks, Deputy Foreign Minister Gennady Gatilov said she was contradicting the common strategy the divided world powers managed to agree at Geneva last week.

Such deadlock in global diplomacy has left the Western powers trying to give an impression of momentum growing against Assad, holding a series of meetings and, as seen on Friday, trumpeting defections and piling psychological pressure on other members of the ruling elite to think about jumping ship.

As Clinton declared in Paris: "Let me say to the soldiers and officials still supporting the Assad regime - the Syrian people will remember the choices you make in the coming days.

"It is time to abandon the dictator, embrace your countrymen and women, and get on the right side of history."

-BBC