SYDNEY (AFP) - A white teenager pictured alongside two Islamic State in Iraq and Syria (ISIS) fighters and dubbed by media as "Britain's white jihadi" is in fact a teenager from Australia who converted to Islam, a report said on Monday.

A picture of the meek-looking youth, holding a rifle and sitting in between two militants with a black ISIS flag in the background, emerged on Twitter in late December.

At the time the militant group, which has run rampant through swathes of Iraq and Syria, hailed his recruitment as "a major coup", with the British media dubbing him "Britain's white jihadi".

Doubts about the authenticity of the picture subsequently emerged after a blogger claimed he had fabricated the image to hoax the British press.

But Australia's Fairfax Media said the photograph had now been positively identified by friends of the teenager and members of two mosques in Melbourne.

It identified him as a former high-achieving 18-year-old student called Jake, declining to reveal his full name at the request of a family member.

He was described as a maths whiz who attended the Craigieburn Secondary College in Melbourne. Friends said he was extremely shy and quiet but was "really bright".

Fairfax Media reported that Jake, who now goes by the Islamic names Abdur Raheem or Abu Abdullah, was 16 when he began feeling unhappy and started searching for spiritual answers. A school friend introduced him to Islam and he recited the verses required to convert.

He dropped out of school in the middle of last year after converting to Islam. Fairfax Media said during his final year in high school, Jake began communicating via online forums and emails with an "American journalist from the BBC". He began telling his family he was going to Istanbul to work with the journalist, even though he had previously expressed little interest in the profession and had no qualifications.

It now appears that the "journalist" was an ISIS recruiter, according to the report.

A source close to the family said in August last year, Jake closed down most of his online and social media accounts and disappeared. A subsequent search revealed that he bought a one-way ticket to Istanbul

"He used to come here when we had a big lecture," Abu Zaid, a committee member of the Hume Islamic Youth Centre in Coolaroo, told Fairfax Media.

"He was a very quiet guy, he stuck to himself. We weren't close to him. I didn't see any of the people (getting) close to him."

The report said that two months after his disappearance, Jake contacted his family to tell them he was in Iraq training for a "martyrdom mission" with a suicide vest.

He later called again to say he was "too scared to do it and he prefers being a soldier" and was planning to travel to Syria.

Around 140 Australians have travelled to fight with ISIS and other terrorist groups in Syria and Iraq, with another 150 supporting them at home, the government has said.

Former immigration minister Scott Morrison said the case showed indoctrination was happening in unexpected places.

"It's very hard to make assumptions on who's going to fall prey to the death cult," he said of ISIS, adding that the government needed "every available tool to stop people joining the fight overseas".

Jake's identification came after Australia stopped two teenage brothers at Sydney airport believed to be heading to the Middle East to fight, amid growing concern in Western countries over young people joining militant groups.

That case followed three British schoolgirls leaving their London homes to join ISIS in Syria in February.