"If we achieve a political transition and it's no longer Bashar in charge of the Syrian army, there could be joint actions against terrorism. But under Bashar it's not possible," Fabius told France Inter radio at the COP21 summit on Monday.
"It is obvious that it's not under the leadership of Mr Assad that the army could be engaged alongside the moderate opposition," he said.
France has long insisted on Assad’s removal. But there had been signs it might moderate its position towards Assad as its priority shifted to tackling the Islamic State armed group which carried out attacks in Paris on November 13.
Fabius fuelled the rumours, by telling France's RTL radio last week that “regime forces” could potentially join the fight against IS armed group. His comments made headlines around the world, even after Fabius said he was merely referring to a post-Assad regime.
Germany backed Fabius's calls over the weekend, with Defence Minister Ursula von der Leyen saying she was open to working with "part of the troops in Syria".