Paris prosecutors opened a preliminary war crimes inquiry on September 15, a source close to the case told AFP. A diplomatic source confirmed the launch of the probe.
The investigation is focusing on evidence provided by a former Syrian army photographer known by the codename "Caesar," who defected and fled the country in 2013, bringing with him some 55,000 graphic photographs of scenes from the brutal conflict.
The announcement comes as the four-year war in Syria takes centre-stage at the United Nations General Assembly in New York, where US President Barack Obama and his Russian counterpart Vladimir Putin have clashed over how to bring an end to the crisis.
French President Francois Hollande has joined Obama in insisting Assad cannot play a role in the country's future, against opposition from Damascus's allies Russia and Iran.
"Russia and Iran say they want to be part of a solution," Hollande said.
"So we must work with these countries to explain to them that the route to a solution does not go through Bashar al-Assad."