He said IS would fight until the black flag was flying over Buckingham Palace and the White House.
"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," he said.
"Bring your planes. Bring everything you want to us. Because it will not harm us. Why? Because we have Allah."
This 17-year-old Australian fighting with the Islamic State (IS) group has appeared in a propaganda video in which he tells Prime Minister Tony Abbott the terrorist group will never stop fighting.
The speaker is believed by Australian security agencies to be Abdullah Elmir, a 17-year-old Australian who went missing from his home in Sydney’s west at the end of June.
In the video he delivered a message specifically mentioning Mr Abbott and Australia.
"To the leaders, to Obama, to Tony Abbott I say this; these weapons that we have, these soldiers, we will not stop fighting, we will not put down our weapons until we reach your lands, until we take the head of every tyrant and until the black flag is flying high in every single land," he said.
WATCH THE VIDEO HERE on ABC News Australia's site.
