Family of Australian teen fighting with ISIL says he is a brainwashed 'idiot'

The family of a Sydney teenager fighting in Iraq with Islamic State militants have condemned his actions and called him a "stupid idiot".

Abdullah Elmir, 17, appeared in an ISIL propaganda video threatening Australia and Prime Minister Tony Abbott, four months after he was reported missing from his Bankstown home.

An unnamed relative of the teenager told the Daily Telegraph he had no idea what he had gotten himself into.

"What a stupid idiot," he said.

"We never associated with him, that guy was brainwashed.

"They don't know how to fight…they should be getting an education and building this country up."

Dressed in military uniform and waving a rifle, the young jihadist speaks for a little under two minutes, vowing to fight until he has raised the black flag of ISIL above the "beheaded tyrants" of world leaders.

"This message I deliver to you, the people of America. I deliver this message to you, the people of Britain, and I deliver this message to you, especially, the people of Australia," he said.

"Bring every nation that you want to come and fight us. It means nothing to us. Whether it's 50 nations or 50,000 nations, it means nothing to us. Bring your planes. Bring everything you want to us. Because it will not harm us. Why?

Because we have Allah, glorious be he."

He continues: "To the leaders, to Obama, to Tony Abbott, I say this: these weapons that we have, these soldiers, we will not stop fighting."

The Federal Government is understood to be investigating the video.