France has identified a second national who appeared unmasked in a grisly Islamic State execution video.
It comes after French jets in Iraq pummelled trenches used by the fighters in a fresh round of air strikes.
French Defence Minister Jean-Yves Le Drian announced that six Mirage fighter jets would be sent to Jordan in December to boost air forces.
The move is part of a US-led international campaign to provide air support to Iraqi and Kurdish forces fighting the Islamic State group.
On Tuesday night French jets targeted IS trenches around the northern Iraqi city of Kirkuk, Le Drian said.
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While France planned to step up its campaign abroad, at home it reeled from the news that two of its nationals appeared in an IS video featuring the execution of 18 Syrian prisoners and US aid worker Peter Kassig.
IS has carried out widespread atrocities since seizing control of large parts of Iraq and Syria, executing five Western hostages and hundreds of locals.
Several of the jihadists appeared unmasked in the latest execution video and one foreigner, 22-year-old Maxime Hauchard from Normandy in northern France, was quickly identified by French prosecutors.
French prosecutors on Wednesday said there was "precise and consistent evidence" that a second national in the video was 22-year-old Mickael Dos Santos from an eastern Paris suburb, who goes by the name Abu Othman.
French authorities estimate around 1000 nationals have taken part in the conflict, with 375 currently in the country.
Valls said "close to 50" French people had been killed in Iraq and Syria.
"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.
Thousands of foreign fighters have flocked to join IS in Iraq and Syria, and experts say they are often among the most violent and brutal of the jihadists.
A British-accented militant nicknamed "Jihadi John" has been at the centre of previous IS beheading videos and appeared again in Sunday's recording.
Other known foreign fighters are believed to have appeared in the video, including an Australian and a Dane.
French President Francois Hollande, on a visit to Australia, said the issue of foreign fighters and how they were being "brainwashed" was a major concern.
"They could be from any background, from any ethnic origin - but they can easily be brainwashed into becoming converts, and this is a very important matter," he said.
