DAMASCUS — A video posted online by “Islamic State” militants on Tuesday showed a boy fatally shooting Muhammad Musallam, an Israeli Arab accused by the group of having signed up as a jihadi to spy for Israel’s intelligence service.

The video, published by the group’s Furqan media outlet, showed Musallam, 19, sitting in a room wearing an orange jumpsuit, talking about how he had been recruited and trained by Mossad. He said his father and elder brother had encouraged him.

After that, it showed Musallam being escorted to a field and then being shot in the head by a boy, described by an older, French-speaking fighter as one of the “cubs of the caliphate.”

Israeli security officials said they were aware of the video but could not confirm that it was authentic.

The video, about 13 minutes long, showed Musallam on his knees when the boy armed with a pistol and wearing a military uniform fires one bullet into Musallam’s forehead. The boy then fired three more shots into his body.

In the video, Musallam said he had joined “Islamic State” to report to the Israelis on weapons caches, bases and Palestinian recruits. Israel and his family denied that he was an Israeli spy.

An Israeli security official had said Musallam went to Syria to fight for “Islamic State” in October last year.

Musallam’s father has denied his son was a spy, saying a month ago that his son had gone missing while on a tourist trip to Turkey.