A young Frenchman and two young men from Wales and England are believed to be among the cold-eyed fighters on an Islamic State propaganda video showing a beheaded American aid worker and the deaths of more than a dozen Syrian soldiers, officials said Monday.

President Barack Obama confirmed the slaying of American aid worker Peter Kassig after a U.S. review of the video, which also showed the mass beheadings of more than a dozen Syrian soldiers.

The overwhelming majority of Islamic State 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 Islamic State supporters, with officials saying thousands of young Europeans have headed off to jihad.

French Interior Minister Bernard Cazeneuve said Monday that a French citizen who joined Islamic State extremists 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," Cazeneuve said, using an Arabic acronym for the Islamic State group. "These are criminals that are building a system of barbarity."

Cazeneuve said authorities were analyzing the video and have been investigating Hauchard, who is around 22 years old and from west of Paris. The convert to Islam gave an interview to France's BFM television in July, telling the network he had helped capture Mosul, the Iraqi city whose fall eventually prompted the United States to resume military operations in Iraq.

One father from Wales, Ahmed Muthana, said he thinks he saw his son Nasser Muthana, 20, in the latest video, and a British researcher confirmed the likeness of the jihadi with the Cardiff medical student.

"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 told Britain's Press Association.

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."

The 26-year-old Kassig, 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 in a statement.

"(Our son) lost his life as a result of his love for the Syrian people and his desire to ease their suffering," the slain hostage's parents, Ed and Paula Kassig, said in a statement.

With Kassig's death, the Islamic State group has killed five Westerners it was holding. Unlike previous videos of slain Western hostages, the footage released Sunday did not show the decapitation of Kassig or the moments leading up to his death, or threaten to kill any other Western hostages.

The footage released Sunday identified the militants' location as Dabiq, a town in northern Syria that the Islamic State group uses as the title of its English-language propaganda magazine and where they believe an apocalyptic battle between Muslims and their enemies will occur.

The high-definition video also showed the beheadings of about a dozen men identified as Syrian military officers and pilots, all dressed in blue jumpsuits. The black-clad militant warns that U.S. soldiers will meet a similar fate.

"We say to you, Obama: You claim to have withdrawn from Iraq four years ago," the militant said. "Here you are: You have not withdrawn. Rather, you hid some of your forces behind your proxies."

A U.S.-led coalition is targeting the Islamic State group in airstrikes in northern Syria, supporting Western-backed Syrian rebels, Kurdish fighters and the Iraqi military.

Kassig, who served in the U.S. Army's 75th Ranger Regiment, a special operations unit, deployed to Iraq in 2007. After being medically discharged, he returned to the Middle East in 2012 and formed a relief group, Special Emergency Response and Assistance, to aid Syrian refugees.

The terror group still holds other captives, including British photojournalist John Cantlie, who has appeared in several videos delivering statements for the IS, likely under duress, and a 26-year-old American woman captured last year in Syria while working for aid groups. U.S. officials have asked the woman not be identified out of fears for her safety.

The group's militants have beheaded and shot dead hundreds of captives, mostly Syrian and Iraqi soldiers, during its sweep across the two countries, and has celebrated its mass killings in extremely graphic videos.

The Islamic State group has declared a self-styled Islamic caliphate in areas under its control, which it governs according to its violent interpretation of Shariah law, including massacring rebellious tribes and selling women and children of religious minorities into slavery.

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Associated Press writers Gregory Katz in London, Diaa Hadid in Beirut, Julie Pace in Brisbane, Australia, David Aguilar in Detroit, Jon Gambrell in Cairo, Vivian Salama in Baghdad and Josh Lederman aboard Air Force One contributed to this report.