A Sydney man believed to be fighting in the civil war in Syria has issued a death threat against a critic of the extreme militant movement he supports in the conflict, ISIS.

"Wait till I send some1 to ur house," the Facebook post warned. "I'll make sure you pay for what u say."

"Dnt worry I know all about u were u live ur not hard to find its just a matter of time... I'll get some1 to put u away for good."

The menace-laden message was sent from a Facebook nom de guerre Abuhafs Al Australi, a profile that until it was removed on Monday night, displayed photographs of its presumed owner, Sydney man Mohamed Elomar.

Zacky Mallah, an outspoken young Muslim of Lebanese origin who lives in western Sydney, was the recipient of the message.

"I laughed at it," he said. "I've had a lot of death threats."

When the Syrian civil war erupted, Mr Mallah, like many in the Middle Eastern diaspora, was quick to side with the rebels.

It is hardly surprising given the brutal record of Syrian president Bashar al Assad.

Mr Mallah still refers to Mr Assad as "the tyrant dictator". But after visiting the war zone to see first hand the carnage inflicted largely upon civilians, Mr Mallah's views changed.

On return to Sydney, he began to question the more extreme elements of the uprising. He is fiercely opposed to one group in particular - ISIS.

"This is an extremist group," Mr Mallah said. "It's even too extreme for Al Qaeda itself."

Mr Mallah is not one for holding back. In a forthright condemnation of ISIS posted to YouTube a few weeks ago, the young activist denounced the group in explicit terms.

Mallah refuses to back away from ISIS criticism, despite threat

Even now, after a death threat in reply from Abuhafs, Mr Mallah refuses to back away.

"I have no sympathy for these people (who support ISIS), I have no sympathy for their families who send them to support ISIS," Mr Mallah said.

"And quite frankly I won't send my condolences to any Australian family who supports ISIS."

It is perhaps little wonder that the person behind the Abuhafs Facebook page took offence at Mr Mallah's remarks, for Abuhafs is a conspicuous ISIS enthusiast.

Among the many video and written endorsements for all things ISIS on the now-defunct profile, Abuhafs makes tasteless remarks on snapshots of dead men killed in fighting with ISIS forces.

Over a photograph of a corpse with soiled trousers is this teasing caption: "Maliki soldiers need to wear nappies before visiting ISIS."

It is a reference to Nuri al Maliki, the Iraqi prime minister whose forces are waging war on ISIS in Iraq.

ISIS ambitions range well beyond Iraq. It is the vision of the movement that one day an ISIS-controlled Islamic caliphate will exist across current day Syria, Iraq and Lebanon. It is dreaming big for a movement that has been sidelined, even by Al Qaeda.

If Mr Mallah has his way, ISIS will fail and fade into obscurity, and so will the threatening bully behind Abuhafs al Australia.

"I don't know him personally, but people have a beef with me," Mr Mallah said.

He has not reported the threatening Facebook message to police as he reckons the authorities who watch social media have already taken note of it.

"I am sure that the police are monitoring all of us," he said.

It is understood police from several agencies had been keeping a close eye on the Facebook profile of Abuhafs Al Australi.

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The authorities are convinced that Mohamed Elomar, the man whose face was plastered across the site brandishing weapons of war, was operating the profile until it was shut down on Monday night after the ABC identified him as the husband of the first woman to be charged under the Foreign Incursions Act.

Fatimah Elomar was revealed on Monday as the mystery Brisbane mother of four stopped by police at Sydney Airport on May 3 while attempting to board an international flight.

She is due to appear in court in Sydney on June 2 on charges of supporting terrorism. If convicted she faces a jail term of up to 10 years.

A lawyer for Elomar insists the trip was nothing more sinister than an overseas holiday to reunite the young family with Mohamed Elomar.

Mr Mallah insists he is unworried. Quite the reverse. Knowing that he moves in the same circles as Elomar's friends in western Sydney, Mr Mallah relished the opportunity to re-state his denunciation of the ISIS movement.

"I wouldn't say there are a lot of supporters here," Mr Mallah said, adding that those who are "are extreme, hardline, narrow-minded and ... bloodthirsty."

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