This follows an earlier Reuters report, citing two senior Syrian officers, that Bashar al-Assad had agreed to a "peaceful" transfer of power after negotiations and boarded a Syrian Air plane at Damascus airport early on Dec. 8.
After flying toward two key Russian military bases - the Hmeimim airbase and the Tartus naval base on the Mediterranean coast - the plane made a sharp U-turn over Homs and lost signal.
Clashes between forces loyal to Syrian dictator Bashar al-Assad and opposition armed groups began on Nov. 27 in the rural areas of Aleppo province in northern Syria. The offensive developed rapidly, with rebels capturing city after city.
On the morning of Dec. 8, opposition forces entered Damascus without a fight.
Bashar al-Assad has ruled the country for 24 years, succeeding his father Hafez al-Assad, who seized power in a military coup in 1970.
A revolution began in Syria in 2011, which escalated into a civil war involving the opposition, radical Islamists, and the Assad regime, which was supported by Russian troops.