A Syrian general close to President Assad has defected, delivering "a hard blow for the regime", French Foreign Minister Laurent Fabius has said.
Brig Gen Manaf Tlas fled Syria via Turkey, his family confirmed.
Mr Fabius said his departure showed Mr Assad's entourage was realising the regime was unsustainable. US Secretary of State Hillary Clinton spoke of "insiders voting with their feet".
If confirmed, it would be the highest-level defection since the unrest began.
Pro-government website Syriasteps said earlier Gen Tlas had made an "escape", adding the move was "insignificant".
Gen Tlas, believed to be in his mid-40s, is a commander of a unit of the elite Republican Guard. As a young man he attended military training with President Assad.
Speaking after a Friends of Syria meeting in Paris, Mr Fabius described him as a "personality who belongs to the Republican Guard of Bashar al-Assad and was for a long time was one of his friends and close to him".
"Even those close to Assad have begun to understand that one cannot support a slaughterer like Bashar al-Assad."
Initially, Mr Fabius said the general was on his way to Paris, but later added he had no indication of his final destination.
The general's father, former Defence Minister Mustafa Tlas, reportedly lives in France.
US Secretary of State Hillary Clinton, also at the Paris meeting, said that if the "increasing stream of senior military defectors" was any indication, then "regime insiders and the military establishment were starting to vote with their feet".
The violence continued on Friday, when Syrian forces captured the northern city of Khan Sheikhoun from rebels. Activists said 25 people were killed.
With the UN observer mission's mandate in Syria due to expire in two weeks, UN Secretary General Ban Ki-moon called on the Security Council to reduce its size and shift its focus from monitoring to political mediation.
The BBC's Barbara Plett, at the UN in New York, says by changing the focus of the mission rather than withdrawing it, the UN would be sending the message that the international community is not abandoning Syria.
Although Gen Tlas's intentions are not yet clear, Amer al-Sadeq, a member of a Damascus-based opposition group, described the latest development as "a good sign".
"Defecting soldiers, we see many of them, defecting officers, the more they come the better it is to make the regime weaker," Mr Sadeq told the BBC.
Gen Tlas has been under a form of house arrest since May 2011 because he opposed the security solution that the regime has been implementing, sources say.
He was also the first government official to meet the opposition last year to try to start a dialogue and find a political solution to the 16-month crisis.
Unlike most of Syria's leaders, who are Alawites, Gen Tlas is a Sunni Muslim.
Syria's majority Sunni community has been at the forefront of the revolt against the president and has borne the brunt of the state's crackdown.
But the Tlas family has given support to the Assad family for decades, helping to ensure Bashar al-Assad's succession to the presidency 12 years ago.
News of his desertion coincided with a Friends of Syria conference in Paris where 107 countries called for tougher sanctions on Damascus.
The group urged the UN Security Council to adopt urgently Mr Annan's six-point plan under an article of Chapter 7 of the UN Charter that refers to economic, diplomatic, travel and communication measures.
Russia and China, which did not attend the meeting, both hold vetoes in the Security Council.
Mrs Clinton urged the representatives to persuade Russia and China "get off the sidelines" and end their support for the Syrian regime.
"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."
Moscow rejected her remarks as "inappropriate".
The Paris meeting followed similar events in Tunis and Istanbul which demanded tougher action against the Assad regime.
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''.
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.
Some 15,800 people have died in more than a year of violence in Syria, activists say.