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The image, taken in the northern Syrian city of Raqqa, was posted on the Twitter account of Khaled Sharrouf, a Sydney man who fled to Syria last year and is now an Islamic State fighter, News Corps reported on Monday.
The photo is believed to be that of his son and was posted with the words, Thats my boy!
The boy, wearing a cap, checked pants and a blue shirt, struggles with both arms to hold up the head of the slain soldier.
It is one of a several photos posted by Sharrouf, who security agencies believe travelled to Syria with his family. One shows Sharrouf also holding the decapitated head.
Another photo shows Sharrouf dressed in camouflage fatigues and posing with his three young sons who are holding guns, the flag of the Islamic State behind them.
They appear to be aged around four, six and seven, News Corps reports.
Sharrouf, a convicted terrorist, is wanted by Australian Federal Police over crimes in Syria and Iraq, which include the shooting execution of a captured Iraqi official in the desert outside the Iraqi city of Mosul.