Decked out in his US army-issued fatigues and a lip stud shining from his mouth, the young American fighter cuts an unusual figure in the northern Iraqi town of Al-Qosh.

He served in the US army in Baghdad in 2006-2007 and has now returned to fight the Islamic State jihadist group with Dwekh Nawsha, a Christian militia whose name is an Assyrian-language phrase conveying self-sacrifice.

The 28-year-old, who goes by the pseudonym Brett, has become the figurehead of an emerging movement of foreigners coming to Iraq to support Christian groups.

Bearing a tattoo of a machinegun on his left arm and another of Jesus in a crown of thorns on his right, Brett jokingly refers to himself as a "crusader".

IS never captured Al-Qosh -- but it came close enough for its mostly Christian population to flee to the neighbouring autonomous region of Kurdistan, together with tens of thousands from Mosul and the Nineveh plains.

"One man's terrorist is another man's freedom fighter," Brett says, speaking from a Dwekh Nawsha base in the Kurdish city of Dohuk.

"But here we're actually fighting for the freedom of the people here to be able live peaceably, to be able to live without persecution, to keep the church bells ringing."

The mass exodus that took place in mid-2014 has put the continued existence of one of world's oldest Christian communities into question.

With Kurdish peshmerga fighters now clawing back land around Mosul, some Christians are keen to take up arms for their survival and Dwekh Nwasha is only one of several recently formed groups.

- 'Foreign fighters' battalion' -

Also acting as a recruiter, Brett says he wants to establish a "foreign fighters' battalion".

In his first week in charge, he brought in five volunteers from the United States, Britain and Canada, all of whom he says have military or contracting experience.

The foreign contingent is tiny compared to the thousands of foreigners who have joined IS, but interest is growing and Brett says he has 20 more volunteers already lined up to join.

Brett's first recruit was Louis Park, a mild-mannered Texan who retired from the Marines in December.

"I did not adjust well at peace time," he said with dipping tobacco tucked in his lip. "I wanted to get back out here."

After serving in Afghanistan, Park says he was diagnosed with post-traumatic stress disorder "and some other things" that barred him from combat deployments.

As early as October 2014, he began saving money to join the fight against IS.

Park says he travelled to Iraq to continue defending his country, even though Dwekh Nawsha -- with barely a few hundred fighters in its ranks -- sees little frontline action.

"I'm patriotic as hell," he says. "If my government won't fight them I will."

The growing contingent of foreign recruits have a variety of reasons for joining Dwekh Nawsha.

Andrew, an older man from Ontario, Canada, came because he heard about "slaughterhouses" where IS allegedly cuts people up for organ trafficking.

There is no evidence that such places exist but the rumour has been widely circulated by evangelical and anti-Islam organisations, especially in North America.

A video showing the beheading of 21 Coptic Christians by IS in Libya released on Sunday and entitled "A message signed with blood to the nation of the cross" sparked a fresh surge of calls on social media for tougher Western action.

- 'Internet cowboys' -

One seven-year US army veteran called Scott says he was planning to join the Syria-based Kurdish "Popular Protection Units" (YPG) until he found out they were "a bunch of damn Reds".

Other foreigners in Dwekh Nawsha say they were turned off by what they see as the socialist streak in the YPG, an affiliate of Turkey's Kurdistan Workers' Party whose months-long battle against IS in Kobane attracted many volunteers.

Alan Duncan, a prominent British foreign fighter and veteran of the Royal Irish Regiment, recently left the YPG for similar reasons.

He told AFP that an exodus of foreign fighters from the YPG has begun, naming several well-known volunteers currently fighting for the group he says plan to leave in the coming days.

Jordan Matson, a former US soldier who has become the poster boy of YPG foreign fighters, argued that some volunteers may have lost their bottle when confronted with the intensity of the fighting in Kobane.

"Most of the Internet cowboys have come to realise this isn't a normal deployment," he told AFP. "So they lose the stomach to come or stay."