France has named a second Frenchman in a video showing the killing of Syrian prisoners by Islamic State (IS) militants as Mickael Dos Santos, 22.
The Muslim convert comes from an eastern Paris suburb. The other French militant was named as Maxime Hauchard, a convert from Normandy.
In the same video, the severed head of US hostage Abdul-Rahman Kassig was displayed for the camera.
About 1,000 French jihadists are thought to have gone to Syria and Iraq.
French Prime Minister Manuel Valls said after a security meeting in Beauvais, north of Paris, that "close to 50" French citizens had died in Syria.
"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.
Meanwhile, new IS video has appeared which shows a small group of gunmen burning what appear to be French passports before three of them make appeals to French Muslims to abandon France and join IS, or stay there and attack "enemies of Allah".
It is unclear where the video was filmed but it was released by al-Hayat, the media arm of IS.
In other developments:
- US Central Command said it had carried out six air strikes on IS targets in Syria since Monday and been involved in 24 coalition strikes on IS in Iraq in the same period
- France said it was sending six Mirage jets to Jordan to carry out air strikes against IS in Iraq
'Kill any civilian'
Confirmation of the second French militant's identity came from the Paris prosecutor's office after unofficial reports in France's media.
France 2 (in French) quoted a friend of the militant who recalled how he had surprised his family when he suddenly converted to Islam.
Of Portuguese origin, he was born in the French town of Champigny-sur-Marne and is believed to have left for Syria in the autumn of 2013.
French intelligence became aware of him after he published an online video in October calling for "all brothers living in France" to "kill any civilian" in retaliation for air strikes carried out by France against IS in Iraq.
French media review - BBC Monitoring
Most news sites lead on the story with Liberation asking, "What is known about the second French jihadist in the video?"
Le Figaro describes Mickael Dos Santos as a recent convert to Islam who had adopted the nom de guerre Abu Othman.
According to Le Monde, more than 1,100 French nationals may be involved in jihadist activity in Iraq and Syria, with more than 375 active in front-line fighting.
Le Figaro features a video on its front page of a young Frenchman who recently converted to Islam. He says that he was approached by Islamic State to join in their jihadist campaign as soon as he converted.
French President Francois Hollande said officials had not established the "exact role" of the two militants.
He called for families to be given more information about the danger of jihadist websites and urged families to be "vigilant" in stopping young people from being recruited by extremists.
Earlier this week, Mr Hauchard was named by a French prosecutor as one of those leading Syrian prisoners to their deaths.
In the latest IS video - unlike previous ones showing beheadings - several militants appeared with their heads uncovered.
The footage showed 18 Syrian prisoners, described as soldiers, forced to kneel in front of the militants before being beheaded.
The men were described as pilots and officers loyal to Syrian President Bashar al-Assad, Reuters news agency reported.
The video also shows the severed head of Abdul-Rahman Kassig, a US aid worker who was kidnapped in Syria last year.
IS said the video was shot in the Syrian town of Dabiq, which features in Islamic prophecies as the site of a final battle between Muslims and their enemies.
Concerns about the involvement of French citizens in the conflict were heightened after a gun attack earlier this year on a Jewish museum in Brussels.
The attack, in which four people were killed, was carried out by 29-year-old Mehdi Nemmouche, a Frenchman who had fought as a jihadist in Syria.
France has the largest Muslim community in the European Union, and is thought to provide the biggest contingent of Western jihadists in Syria and Iraq.
More than 100 French citizens fighting in the Middle East have already returned to France, with the vast majority now facing charges under terrorism legislation, the BBC's Lucy Williamson in Paris reports.
Earlier this month, France convicted its first such returnee with a jail term of seven years, our correspondent adds.
The government has also given police new powers to confiscate passports, to prevent people travelling abroad to join militants.