Maxime Hauchard, a French jihadist sought by French and American authorities since emerging in an Islamic State (ISIS) execution video in Syria, has been killed, sources close to the inquiry told AFP on Thursday.

"The date and circumstances of his death aren't yet known, but it appears he died in the summer of 2017," one of the sources said.

Hauchard, who grew up in a village in Normandy before converting from Catholicism to Islam, was 22 years old when he was seen in an ISIS video showing the beheading of U.S. aid worker Peter Kassig in November 2014.

The video also showed the execution of 18 Syrians identified as military personnel.

A second French national who appeared in the same video was later identified as 22-year-old Mickael Dos Santos from an eastern Paris suburb, who goes by the name of Abu Othman. His fate is not known at this time.

France issued an international arrest warrant for Hauchard, and the State Department added him to its black list of "specially designated global terrorists".

Investigators later found that Hauchard became radicalized online, joining jihadist forums under the moniker Abu Abdallah al Faransi ("the Frenchman").

He twice travelled to Mauritania between October 2012 and May 2013 for studies in Salafism, the highly conservative branch of Islam, according to AFP.

In August 2013 he left for Syria via Turkey, telling his family he wanted to "help the wounded" in the country's civil war, but in fact he was taken under the wing of ISIS recruiters.

A few months later ISIS released the video showing the execution of Kassig and the Syrian soldiers, in which Hauchard was shown with his face uncovered.

He emerged again in November 2015, a few days after the Paris terror attacks that killed 130 people, warning on Twitter: "Brazil, you're our next target," a reference to the 2016 Summer Olympics in Rio de Janeiro.

The French government says about 1,700 French nationals have left to fight alongside ISIS in Iraq and Syria since 2014.

France is not the only country that has dealt with radicalization. Another group of four Europeans infamous for their role in ISIS’ executions of Western nationals came from Britain and were known as the “Beatles” because of their British accents.

Last month it was reported that Syrian Kurdish fighters had detained two members of the “Beatles”. Officials identified the two men captured as Alexanda Kotey and El Shafee Elsheikh. They were the last two members of the group to remain at large.

The group's leader, Mohammed Emwazi who was known as “Jihadi John, was killed in an airstrike in 2015 in Syria after an intensive manhunt. “Jihadi John” was seen in ISIS videos showing the beheadings of journalists James Foley and Steven Sotloff, British aid workers David Haines and Allan Henning, and Japanese hostage Kenji Goto.

A fourth member of the so-called “Beatles”, Aine Davis, is imprisoned in Turkey on terrorism charges.