An activist group has said France's first air strike on Islamic State (IS) militants in Syrian territory killed 30, including 12 child soldiers.
The UK-based Syrian Observatory for Human Rights said Sunday's strike on an IS camp in eastern Syria had also wounded about 20 fighters.
French President Francois Hollande said on Sunday six jets had destroyed the camp in the province of Deir al-Zour.
France had previously limited its strikes on IS to Iraqi airspace.
French Defence Minister Jean-Yves Le Drian said the raid had struck "an extremely sensitive site for [IS]", describing it as a "strategic hub" for militants travelling between Iraq and Syria.
He said the raid was carried out in self-defence, saying that France had evidence that foreign fighters were being trained in the camps to carry out attacks in Europe and in France specifically.
A teenaged former fighter told the BBC in June that he had been trained to fight for IS alongside mostly "15 and 16 year olds".
"There were even many as young as 13 or 14. Those are more eager to fight and wage jihad for the sake of God," he said.
The UN has documented the use of children in combat and support roles by both pro- and anti-government forces in Syria, as well as by Islamic State.
'Bureaucracy of horror'
Meanwhile, on Tuesday evening French Foreign Minister Laurent Fabius said French prosecutors were opening an investigation into alleged crimes against humanity committed by the government of Syrian President Bashar al-Assad.
The investigation will focus on a report published last year by three former war crimes prosecutors which accused the Syrian government of systematically torturing and executing about 11,000 detainees since the start of the uprising against Mr Assad's rule in March 2011.
That report was based on the evidence of a defected military police photographer, referred to only as Caesar, who along with others reportedly smuggled about 55,000 graphic digital images of dead detainees out of Syria.
"Faced with these crimes which offend the human conscience, this bureaucracy of horror... it is our responsibility to act against the impunity of these killers," Mr Fabius said.