French foreign minister Laurent Fabius says the defection of a Syrian general close to President Bashar al-Assad is a hard blow for the leader's regime.
Brigadier-General Manaf Tlas fled Syria via Turkey and is believed to be on his way to Paris.
If confirmed, it would be the highest-level defection since the unrest began, the BBC reports.
Mr Fabius said the departure of General Tlas showed Mr Assad's entourage was realising the regime was unsustainable.
US Secretary of State Hillary Clinton 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.
Pro-government website Syriasteps said earlier General Tlas had made an "escape", adding the move was insignificant.
The general, 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.
Violence continued in Syria on Friday when government forces captured the northern city of Khan Sheikhoun from rebels. Activists said 25 people were killed.
Call for smaller UN mission
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.
In a report to the security council, Mr Ban suggests the observers should be based primarily in the capital, Damascus, and use contacts with both the government and the opposition groups to foster dialogue.
Countries supporting the uprising have urged the UN to ratchet up pressure on President Assad, by threatening his regime with tough sanctions.
Representatives of the Syrian opposition have also called for humanitarian corridors and the creation of a no-fly zone.
At the same time, the UN Secretary General, Ban Ki Moon, has recommended the number of UN observers in Syria be reduced.