According to Ms Katz, the video carries on a recent trend of IS members targeting Christians in their attacks "but (is) also a throwback to its older propaganda from Iraq (and) Syria of brainwashed children carrying (out) grisly executions".
She said the video showed "a repeat of the same indoctrination". According to Ms Katz and SITE, the child perpetrating the attack was approximately eight years old, and the killing took place outdoors in the state of Borno in northeast Nigeria in Islamic State's West Africa Province (ISWAP).
The victim has not been formally identified, but SITE said he was a Nigerian man and a Christian. Reports have suggested he is a reverend who was captured by IS fighters at the beginning of January.
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Late last year a group linked with IS in Nigeria claimed responsibility after 11 people were brutally executed on video.
The video, released in late December, showed 10 people having their throats cut, while another person was shot. A voiceover in the video warned the killings were a "message for Christians" and claimed all those killed in the video were Christian.
According to reports, the group claimed the slayings were in retaliation to the US killing of IS leader Abu Bakr al-Baghdadi in Syria.
"He was a sick and depraved man, and now he's gone," US President Donald Trump. "He died like a dog, he died like a coward."
The ISWAP is a splinter group from the larger Boko Haram jihadi terrorist group based in Nigeria, with influence in neighbouring countries. Crisis International suggested the group had between 3500 and 5000 members in May of last year.