A young Frenchman and two men from Wales and England are believed to be among the cold-eyed fighters on an Isis propaganda video showing a beheaded American aid worker and the deaths of more than a dozen Syrian soldiers.
President Barack Obama confirmed the slaying of American aid worker Peter Kassig after a US review of the video.
The majority of Isis fighters are from the Mideast, but as the extremist group tries to cement its claim on an Islamic empire straddling Iraq and Syria, it is increasingly trying to portray itself as an international movement.
Europe appears to be a fertile ground to find supporters, with officials saying thousands of young Europeans have headed off to jihad.
French Interior Minister Bernard Cazeneuve said that a French citizen who joined Isis in 2013 is believed to be among the knife-wielding extremists lined up behind their kneeling victims in the latest video.
The camera lingered on the faces of the militants, identified as Frenchman Maxime Hauchard as well as other extremists who seem to come from across the world. Some had distinctly Asian features, while another - whose face was hooded - had the familiar London accent of the jihadi who also appeared in beheading videos with American hostages James Foley and Steven Sotloff, and British hostages David Haines and Alan Henning.
There were also indications that a Welsh medical student may be the man standing alongside Hauchard.
"I call solemnly and seriously on all our citizens, and notably our young people who are the primary target of the terrorist propaganda, to open your eyes to the terrible reality of the actions of Daesh [Isis]," Cazeneuve said. "These are criminals that are building a system of barbarity."
Cazeneuve said authorities have been investigating Hauchard, who is around 22-years-old and from west of Paris. The convert to Islam gave an interview to a French TV station in July, saying he had helped capture Mosul, the Iraqi city whose fall eventually prompted the US to resume military operations in Iraq.
One father from Wales, Ahmed Muthana, said he thinks he saw his son Nasser (20) in the latest video.
"It resembles him. I was shown a picture of the video. I cannot confirm it is him, but I think it might be," Ahmed Muthana said.
Obama denounced the extremist group, which he said "revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction".
Kassig (26), who founded an aid group to help Syrians caught up in their country's brutal civil war, "was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity", Obama said.
Mr Kassig's parents said they are "heartbroken" by their son's death.
Ed and Paula Kassig said they were "incredibly proud" of his compassion and humanitarian work and that he "lost his life as a result of his love for the Syrian people and his desire to ease their suffering".
"We will work every day to keep his legacy alive as best we can," they added.
Unlike previous videos of slain Western hostages, the latest footage did not show the decapitation of Kassig, the moments leading up to his death, or threaten to kill any other hostages.
The video also showed the beheadings of about a dozen men identified as Syrian military officers and pilots, all dressed in blue jumpsuits.
The terror group still holds other captives, including British photojournalist John Cantlie, who has appeared in several videos delivering statements for the Isis, likely under duress, and a 26-year-old American woman captured last year in Syria while working for aid groups.