Fares, the most prominent politician to defect since the uprising against Assad began, insisted that the president's days were numbered but warned he would be prepared "to eradicate the entire Syrian people" to remain in power.
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When asked by the BBC's Frank Gardner whether that would mean the use of chemical weapons, Fares said: "I am convinced that if Bashar al-Assad's regime is further cornered by the people, he would use such weapons."
Syria has a large stock of chemical weapons and neighboring countries are increasingly concerned about what will happen to them if the regime topples.
Fares said this outcome was now "inevitable": "It is absolutely sure that this government will fall in a short time," he told the BBC from his refuge in Qatar. "We wish for this time to be short so that more sacrifices are reduced."
Fares with Assad (Archives: AP)
Fares, who announced his defection on July 11, was widely seen as a regime hardliner and his decision to break ranks has triggered suspicion among activists.
Some dissidents say Fares has been likely groomed by the West to play a role in a transitional government while others have spoken about his "criminal" past.
Fares, who has served as governor in several Syrian provinces and has held senior security and Baath party posts, hails from the prominent Oqaydat Sunni tribe in eastern Syria, which also has members in Iraq, Jordan and Saudi Arabia.
A former policeman, Fares had close ties to the dreaded intelligence services before becoming governor and later Syria's first ambassador to Iraq following a 30-year rupture in ties between the two neighbors.
Syria's military deployed armored vehicles near central Damascus on Monday as troops battled rebels around the capital in what activists said could be a turning point in the 16-month uprising.
Fares said the spread of violence to the capital proved that the "expansion and the power of the revolution was increasing day-by-day."
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