Maxime Hauchard, a French extremist sought by French and US authorities since emerging in an "Islamic State of Iraq and Syria" (ISIS) execution video in Syria, has been killed, sources close to the inquiry told AFP on Thursday (March 15th).

"The date and circumstances of his death are not 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 just 22 when he was seen holding a knife to the neck of US aid worker Peter Kassig in a gruesome video from November 2014.

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

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

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

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.

"To show allegiance, you must first go to a training camp. The first stage lasts around a month. We do some training, we go on operations and after that we return to training. It is not just theory," he told a French TV station in July 2014.

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 are our next target," a reference to the 2016 Summer Olympics in Rio de Janeiro.