The Islamic State terrorist group (ISIS) has executed an Israeli Arab teenager it accused of spying for the Mossad.

In a video of the execution distributed by ISIS, the victim was identified as 19-year-old Mohammed Said Ismail Musallam, an Arab resident of eastern Jerusalem who joined ISIS not long ago.

Dressed in an orange jumpsuit that is standard in videos of ISIS executions, he is shown kneeling in front of a young boy, who appears to be no more than 12 years old, and a man standing at his side.    

The man, speaking in French, issues threats against Jews in France, before the boy walks around in front of the hostage before shooting him in the forehead with a pistol.

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

It is not the first time ISIS has used a child to execute alleged spies; back in January of this year a young Kazakh boy was shown executing two alleged Russian spies.

Speaking before the execution, the adult ISIS terrorist vows his group will conquer Jerusalem, and gloats over January's deadly terrorist attack on a Paris kosher supermarket.

"Oh Jews, indeed Allah has gifted us with killing your followers in your own stronghold in France," he says, referring to the attack by Amedy Coulibaly, who swore allegiance to the group.

"As for the near term, you will see the caliphate pounding on your lands and on your fortresses. They will liberate Jerusalem from your filth, by Allah's permission," he adds.

In a February issue of its magazine Dabiq, the jihadist group claimed Ismail Musallam infiltrated them on behalf of the Mossad, after being enlisted to the Israeli spy agency by a Jewish neighbor.

Allegedly his mission was to infiltrate ISIS and get information on weapons storehouses, as well as to identify Arab ISIS members from Judea and Samaria.

The magazine claimed he was suspected by one of his commanders according to the magazine for unusual behavior, and asked to go home fearing he was exposed at which he was told "it's too late now" and then captured.

However Musallam's father offered a different version of events, telling Haaretz his son did travel to join ISIS several months ago as was reported in the ISIS magazine, and likewise was working as a firefighter after graduating high school as was recorded, but that the claims of his being a Mossad spy are simply lies.

Instead, it appears Musallam may have been one of a growing number of foreign ISIS recruits getting cold feet due to the growing military stranglehold on the group, and attempted to flee the group. Those who are caught are generally accused of spying and executed.

AFP contributed to this report.