Image copyright Reuters

It's been reported that thousands of foreign fighters have made their way to the Middle East to join Islamist militant groups - so-called Islamic State, and others. But how reliable are the figures and how many of the people who went are still there now?

The decision to go to war in a country far from home is not a new phenomenon - in the 1930s about 30,000 foreigners fought in the Spanish Civil War, and the early 19th Century the British poet, Lord Byron, fought for Greek independence from the Ottoman Empire.

In recent times many went to Afghanistan, Pakistan, Iraq, Yemen or Somalia. But the numbers travelling to Syria now represent "the largest foreign fighter mobilisation of Islamist foreign fighters in history" says Thomas Hegghammer, a director of terrorism research at the Norwegian Defence Research Establishment.

There are several reasons for this, he suggests.

Syria is easier to get to than previous conflicts - people have been able to travel to Turkey and cross the border without any difficulty. And once inside Syria, there are areas where the risks are limited. Because IS controls a large territory, foreigners can avoid front-line combat if they want to.

Image copyright AP Image caption A Turkish soldier with an armoured personnel carrier securing a road near the border with Syria

"In the early days of the war, the main declared motivation was, 'I want to go and fight Assad to defend the Sunnis in Syria,'" says Hegghammer. "Now the most common declared motivation is, 'I want to go and live in Islamic State, I want to live in the Caliphate.'

"You have people in Europe who sell all their belongings, they take their children out of school and move to Raqqa with the full intention of living there for the rest of their life."

Interpol has identified 4,000 individuals who have travelled to join the militants. But many put the overall figure of identified and unidentified people, far higher.

"The fact that in December 2013 I was able to identify around 3,800 individuals, suggests that they [Interpol] don't have the full picture." says Aaron Zelin, a fellow at the International Centre for the Study of Radicalisation (ICSR) at Kings College, London, who studied the subject between 2011 and 2013.

"It's been a year and a half since then, and I would argue that the number of foreign fighters has gone up exponentially."

The ICSR now estimates that 20,000 is closer to the truth, a figure it arrives at by monitoring media reports and other publicly available information.

"It can be anything from let's say the UN saying the total number is 15,000 or 20,000 down to the mayor of a town in southern France saying, 'We've had 22 people leave in the past three months,' and everything in-between," says Hegghammer who uses similar techniques.

"The reports are of different types and of different reliability so you need some kind of human subjective assessment in there but I think you can get a reasonably good sense of the scale."

Even so, Hegghammer says the estimate of 20,000 should be taken with a pinch of salt.

"If an analyst or an organisation says, 'We think somewhere between 500 and 900 people have gone from France,' the headline will be almost always be, 'Up to 900 people have left from France,' and then that 900 number kind of sticks and feeds into the aggregation exercise. I think that tends to lead to inflation," he says.

European militants who have gone to Syria/Iraq (estimates) Country Per 1m inhabitants Total Belgium 40 440 Denmark 27 100-150 Sweden 19 150-180 France 18 1200 Austria 17 100-150 Netherlands 14.5 200-250 Finland 13 50-70 Norway 12 60 UK 9.5 500-600 Germany 7.5 500-600 Source:ICSR (Jan 2015)

Also, this is not the number of foreign fighters thought to be waging jihad with IS right now. It represents people who have joined Sunni militant organisations in Syria over the course of the entire conflict. The ICSR estimates that between 10% and 30% - up to 7,000 people - have already returned home and a further 5% to 10% have been killed.

But where do these people come from? According to the UN, more than half of the world's countries are producing jihadist.

The majority are from the Middle East and many come from North Africa. The five countries with the most fighters in Syria are Jordan, Saudi Arabia, Tunisia, Lebanon, and Libya.

Russia has also contributed a substantial number of fighters - with most believed to have come from Chechnya. Another 3,000 come from former Soviet states.

It's estimated that 4,000 are from Western Europe with France, Germany and the UK the biggest contributors. Belgium has contributed more per capita than any other EU country.

But Zelin also agrees with Hegghammer that the figures should be treated with caution.

"I often compare this to sausage making," says Hegghammer. "People say that butchers don't eat sausages because they know what goes into them. And it's the same thing with people like me who work trying to estimate numbers, I rarely trust the estimates that I see thrown around because I know what goes into them."

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