BEIRUT(REUTERS) - A video posted online by ISIS (Islamic State in Iraq and Syria) militants on Tuesday showed a child killing Muhammad Musallam, an Israeli Arab accused by the group of being a Mossad spy, with a bullet to the head.

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

After that, it showed Musallam being escorted to a field and executed by a child, described by an older, French-speaking fighter as one of "cubs of the caliphate".

Reuters could not immediately verify the authenticity of the footage, which also appeared on Twitter feeds used by ISIS supporters.

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

The video, which was about 13 minutes long, showed Musallam on his knees as he listened to the older fighter speaking the verdict in French.

Then the boy, wearing a military uniform and armed with a pistol, stands face to face with Musallam and fires one bullet into his forehead.

Musallam crumbles, then the child shoots him three more times and chants "Allahu Akbar!" (God is Greatest).

In the video, Musallam repeated what he said last month in an interview published by the group's English language magazine Dabiq, in which he 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.

"I tell my father and my son: Repent to God. I say to the spies who spy on Islamic State (ISIS): You will not be successful, they will expose you," Musallam said in the latest video, in Arabic.

Musallam's family have said he went missing while on a tourist trip to Turkey and an Israeli security official has said he went to Syria to fight for ISIS in October last year.