Australian jihadist Abdullah Elmir vows Isis will fight 'until black flag is on the top of Buckingham Palace'

Teenager who ran away in June can be seen surrounded by Isis militants

A 17-year-old Australian boy has appeared in an Isis propaganda video, where he warns the UK and US that militants will keep fighting until “the black flag” is flying over Buckingham Palace.

The video is believed to show Abdullah Elmir, who went missing from his home in Sydney at the end of June after telling his family he was going fishing.

Elmir is understood to have travelled with another 16-year-old Australian boy in June to Turkey and then crossed the border into Syria to join Isis fighters, ABC News reports.

His companion was intercepted by his father and his since returned to Sydney, it has been reported.

In the footage the teenager, who appears to have been radicalised, can be seen directly addressing the Australian Prime Minister Tony Abbott and the US President Barack Obama.

Standing in front of dozens of Isis fighters brandishing weapons, he says: "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.

"Until we put the black flag on top of Buckingham Palace, until we put the black flag on top of the White House, we will not stop, and we will keep on fighting."

The video is the latest in the 'Message from Mujahid' propaganda series released by Isis.

A family member told the Daily Telegraph Elmir may have been “brainwashed”.

“I don’t know what these kids get into,” the unnamed relative said.

“They don’t know how to fight ... they should be getting an education and building this country up.”

In a statement, Mr Abbott said the video "again highlights the threat posed by Isis", who has seized swathes of territory across Syria and Iraq.

"Isis is a threat that reaches out to Australia and our allies and partners," he said.

The Independent has been unable to verify whether Elmir is speaking under any duress.