French police were on Wednesday probing whether a man seen in a video released by the Islamic State (ISIS) group purporting to show the execution of an Arab Israeli was close to French jihadist gunman Mohamed Merah.

In the video, a youth identifying himself as 19-year-old Mohammed Said Ismail Musallam is shown kneeling in front of a boy who appears to be no more than 12-years-old, while a man stands at his side, reports AFP.

Dressed in an orange jumpsuit that is standard in videos of ISIS executions, the man seen kneeling gives a forced confession claiming to have been recruited by Israeli intelligence, a claim denied by his father.

The man standing nearby, speaking in French, issues threats against Jews in France, before the boy walks around in front of the hostage and shoots him in the forehead using a pistol.

The boy, who shouts "Allahu Akbar" ("Allah is greatest" in Arabic), then shoots the man four more times as he lies on the ground.

"We are checking" the identity of the man, a police source who wished to remain anonymous told AFP.

Several experts, including journalist David Thomson who wrote a book about French jihadists, say the man is Sabri Essid, reportedly the half-brother of Merah, who shot dead three soldiers in southern France in 2012 before murdering three Jewish children and a rabbi at a Jewish school in Toulouse more than a week later.

Wednesday marks the third anniversary of the start of Merah's killing spree, which ended with him dying in a shootout with police.

Another source close to the case, who also wanted to remain anonymous, said there were "similarities" with Essid, adding they could not be certain.

Known to intelligence services as a key figure in the radical Islamist community in Toulouse, Essid is suspected to have left France for Syria last year.

Merah's sister Souad also left for Syria last spring, reportedly with family members.

Essid had already been caught in December 2006 in Syria in a house known to shelter Al-Qaeda members on their way to Iraq.

He was sent back to France and was sentenced in 2009 to five years in jail, including one year suspended, in a case involving an Iraqi jihadist network.

His father had lived with Merah's mother, and Essid was close to the killer and his brother Abdelkader.