A government source told AFP the precise number, as of Monday, was estimated at 49.
"We know the number of French nationals, more than a thousand, who are involved in this phenomenon," Valls said after a security meeting in Beauvais, north of Paris.
"We know the number of French who have died in Syria, close to 50, so we know the dangers and, sadly, we are not surprised to learn that French citizens or residents of France are found at the heart of these cells and taking part in this barbarity," he added.
Two French citizens -- Maxime Hauchard and Mickael Dos Santos, both 22 -- have been identified as appearing in this weekend's execution video from the Islamic State group.