The identifications were made by a British father whose son had traveled to Syria to join the Islamic State, and by senior French officials who saw in the video footage a French citizen they had been monitoring for years.
Their assertions seemed likely to deepen apprehension about the role of foreign-born jihadists in the conflict and the possible hazards of their return to native lands.
Thousands of young European Muslims have been reported traveling to join the Islamic State, also known as ISIS or ISIL, in its campaign to revive a caliphate in areas under its control. The battle sharpened significantly in June when the Islamists flooded from Syria into Iraq.
The militant group distributed a video on Sunday showing that it had beheaded a fifth Western hostage, Peter Kassig, an American aid worker and former Army Ranger. The video featured a masked executioner with a British accent who has been nicknamed “Jihadi John” in British media accounts of the earlier executions.
British news reports on Monday said the father of another Briton who had joined ISIS thought the footage included a man who resembled his son. The father, Ahmed Muthana, from Cardiff, Wales, said, however, that he was “not quite sure” that the image showed his son Nasser, who reportedly traveled to Syria in June along with his brother Aseel to join the militants.
“I was shown a picture of the video,” Mr. Muthana said. “I cannot confirm it is him, but I think it might be.”
At the same time, the Paris prosecutor, François Molins, told a news conference in Paris that a Frenchman, Maxime Hauchard, had been identified in the footage.
Mr. Molins said Mr. Hauchard, a 22-year-old from Normandy who converted to Islam at 17, left France for Syria in August 2013 under the false pretext of humanitarian work.
The captives can be seen kneeling on the floor with armed men in black masks standing behind them as one of the militants reads out a statement. Minutes later the four men have been beheaded