"I don't care who they are, Iraqi, Libyan, if they want to help us bring down Assad, then they're all welcome."

"I don't care who they are, Iraqi, Libyan, if they want to help us bring down Assad, then they're all welcome."

This was what one Syrian rebel fighter known only by his nom-de-guerre, Mazzan, told me, when I met him in the hills bordering Turkey two years ago while the Syrian civil war was in its early stages.

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Among the rebels to which Mazzan belonged there were clearly many foreign fighters within their ranks but none, as far as I could ascertain, who were European in origin. How indeed things have changed, and in a very short time.

As the brutal killing of American journalist James Foley has further brought to light, the ranks of jihadist fighters in Syria and Iraq now have a substantial quota of young men from across Europe and beyond. Yesterday, the British security services were escalating their hunt to identify a suspected British jihadist who appeared in footage of Mr Foley's killing.

For some time now those same European security officials have been flagging up an "alarming acceleration" in the number of European jihadists travelling to Syria to obtain combat experience with extremist groups like the Islamic State (IS) and the al-Nusra Front.

The clandestine nature of these various jihadist networks makes it difficult to determine exactly how many foreigners have entered the war and from which countries. That said, data is being collated by a variety of means and the findings make for disturbing reading.

It was as far back as spring last year when the number of foreign fighters surpassed that of any previous conflict in the modern history of the Muslim world. The 1980s war in Afghanistan was the previous record holder for attracting foreign fighters. As I recall from covering the fighting at the time, there was a noticeable rise in the number of foreign fighters that coincided with the withdrawal of Soviet troops in 1989.

On the battlefields around places such as Jalalabad, it was not unusual to bump into Algerian, Chechen, Saudi and other volunteer holy warriors, although today's research now shows there were never more than 3,000 to 4,000 foreign fighters at any one time in Afghanistan.

By contrast, some estimates now indicate there could be more than 10,000 foreign fighters operating in Syria and increasingly in Iraq, where the Islamic State group has recently extended its military campaign.

European officials believe about 2,000 EU citizens have answered the call to jihad.

French intelligence has counted 700 French citizens and foreigners who have gone to Syria from France, while the German interior ministry says 240 of its citizens have volunteered.

Not all those venturing to these frontlines are fully committed jihadists. Many who have travelled to places such as Libya and Syria are nationalists or non-jihadist Islamists. Nevertheless, there are many full blown jihadists among them, along with other Muslims who become heavily influenced by the jihadists they fight alongside.

Most who volunteer are integrated into the al-Nusra Front or Islamic State, but not exclusively. Other groups with concentrations of foreign fighters include the Eagles Of Honour (Suqour al-Izz), founded by Saudi fighters, and the Movement Of Islamic Levant (Harakat Sham al-Islam), led by Moroccan operatives.

These groups aside, perhaps one of the most worrying factors is that as the number of European jihadists in Syria grows, those doing much of the fighting are getting younger and younger.

Earlier this year, the then French Interior Minister Manuel Valls, now Prime Minister, said more than a dozen French nationals under the age of 18 were active as jihadists in Syria. Some too are young women, who have chosen to assist the men in their tasks.

It may also come as something of a surprise that the recruitment of these young people, far from being done through local mosques or militant groups, is instead undertaken mainly via social media and the internet.

According to Thomas Hegghammer, the director of terrorism research at the Norwegian Defence Research Establishment, "Syria is probably the most socially mediated conflict in modern history, and the internet is chock full of propaganda from Syrian jihadi groups, as well as practical travel advice for aspiring foreign fighters."

Not surprisingly then, social media, too, provides some of the best monitoring tools in determining where fighters are actually coming from.

Some researchers closely involved in examining how jihadist groups are adjusting to the new political environment in the era of Arab uprisings, point to clues gleaned from the way jihadists often post death notices for slain fighters as a good way of gauging this.

The notices posted on social media networks, they say, provide a unique, if incomplete, picture of where they are being recruited and where in Syria they fought.

For intelligence analysts, tracking and analysing these notices can help broaden their understanding of foreign recruitment networks.

This snapshot into a murky clandestine world throws up some startling insights and statistics, the most striking of which is the massive increase in overall death notices. By far the vast majority of the more than 1,100 notices of 2013 came in the last six months of that year.

Intelligence gathering research concludes that while nine of the top 10 countries represented are from the Arab world, death notices have mentioned 50 different nationalities in all, including 20 in Europe or elsewhere in the West.

So much then for those who die in combat struggling to establish a caliphate in the wider Middle East. But what of those who survive to return to their countries of origin, many still utterly devoted to their extremist cause?

According to some security sources as many as 260 individuals from Britain who have fought alongside jihadists have already returned home.

The kind of dangers they potentially pose was highlighted in France in 2012 when Mohammed Merah, a French citizen of Algerian descent went on a shooting spree in the French towns of Toulouse and Montauban, targeting French soldiers and Jewish civilians. Mr Merah himself had been radicalised after previously spending time in Afghanistan and Pakistan.

Right now, France, Germany and the UK have among the largest foreign fighter contingents in Syria, but Denmark, Norway, Belgium and Austria have contributed a much higher proportion of their population. If that accumulation of foreign fighters is bad enough for Syria and Iraq then its consequences for the future of international terrorism could be profound.

As for stopping this potential future threat, David Cameron yesterday said there would be a redoubling of efforts "to stop people going, to take away the passports of people contemplating travel, to arrest and prosecute those that take part in extremism and violence."

Yesterday the UK security services were working all out to identify the suspected British jihadist responsible for beheading James Foley. It will be a daunting task, but nowhere near as challenging as staunching the flow of those intent on returning to cause terror at home.