A "mild-mannered" young Arab-Israeli man who ran away to join Islamic State and then had a change of heart has been accused of being a Mossad spy in the extremist organisation's latest propaganda magazine.

Global attention has been focused on the horrors perpetrated by the Islamic State militia on its hostages and those it deems heretics in the land it has conquered.

Relatively little is known about how it handles its own fighters, but an increasing number of cases are starting to emerge, revealing a brutal internal rule.

Now an Arab-Israeli family is dealing not just with the horror that their son has gone to join the militia, but that he has run afoul of their cruel regime.

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Islamic State's Dabiq magazine features an article on 19-year-old Mohammed Msallam, who it accuses of working for Israeli spy agency Mossad.

His parents Hind and Saeed Msallam are reeling in shock.

For this Palestinian family, the accusation is a great insult. For their son, it is probably a death sentence.

The couple said Mohammed asked to borrow about $100 for a training course last August but then disappeared.

When they reported him missing to the police, they were told that he had flown out of the country, bound for Turkey.

His parents said he was a mild-mannered boy who completed his Israeli national service by joining a fire-fighting unit.

The next they heard from him, he had joined the extremists of Islamic State in their de facto capital, Raqqa in Syria.

Islamic State recruit Mohammed Msallam. ( Supplied )

The photos he sent show him dressed in black, festooned with clips of ammunition, holding an assault rifle he could probably barely shoot.

He had been recruited over the internet.

It didn't take long for things to turn sour. In an internet video chat, he asked his parents to send money so he could come home.

"He said, mum, they promised me a house, a wife and a good future," Hind Msallam said.

"They described this place as paradise and when I got there, I had nothing, all of their promises were a mirage."

The family sent $200 to someone in Egypt but heard nothing from Mohammed.

Then a man using a Turkish phone number called with chilling news.

"I was surprise when I talked to this man," Saeed Msallam said.

"He told me on the phone that my son was trying to cross to Turkey, he was arrested by ISIS, now he is captured by ISIS."

The caller said he saw Mohammed in a prison, which held a large number of children, in Tal Abbyad, a Syrian border town south of the Turkish city of Sanliurfa.

Then, last week, the latest issue of Islamic State's propaganda magazine, Dabiq, hit the internet.

It included an article that made the Msallam family fear the worst.

It featured photos of Mohammed in its "Interview with a Spy for the Israeli Mossad" story.

Islamic State propaganda magazine in which recruit Mohammed Msallam is condemned as a spy. ( Supplied )

In it the boy from Jerusalem allegedly detailed his recruitment by his father and brother - allegations they strenuously deny.

His father said his son clearly fell out with the militia.

"This is why ISIS is accusing my Mohammad of being a spy because he wanted to leave them, he escaped and was captured in a checkpoint controlled by ISIS," Saeed Msallam said.

Brandeis University professor Jytte Klausen, who runs a project monitoring foreign fighters, said she knew of a dozens stories of recruits who had fallen foul of their brutal hosts.

"There have been reports of people being executed for trying to leave, children who have been put in jail or in other ways have come to grief because they tried to get home," Professor Klausen said.

"There are also accounts of young people asking their parents to come and get them and then the parents go there and they find that they can't get their children out."