An August 19 video depicting the beheading of U.S. journalist James Foley at the hands of an Islamic State of Iraq and Syria (ISIS) militant with a British accent has raised questions about the increasing profile of the group’s Western fighters.

Since ISIS’s June offensive in Mosul – it now holds a tenuous control over Iraq’s second-largest city – the group has opened the porous border between Syria and Iraq and taken control of the Syrian regime’s air bases in Raqqa, giving it effective control of the province it considers its stronghold.

The momentum, says Peter Neumann, the director and founder of the International Center for the Study of Radicalization and a professor in the Department of War Studies at King’s College London, has led to a 10 to 15 percent increase in foreign fighters coming from Europe in the past two months – an additional 200 to 300 men.

But he says tensions are on the rise between the new Western recruits – many of whom lack battlefield experience and Arabic-language skills – and ISIS’s Syrian fighters, who say the group’s more brutal activities have no basis in their country’s history or culture.

Here, Neumann discusses the emerging hierarchy and roles played by Western fighters in Syria and Iraq, and why events of the past two months have galvanized so many young Europeans to make the journey.

Syria Deeply: Is there tension between Syrians and the increasing number of foreign fighters?

Peter S. Neumann: We hear from a number of Syrians that the foreign fighters are not that popular, and that a lot of Syrians do not like them very much. There are differences between Syrian and foreign fighters when it comes to social norms, how to practice Islam, which punishments to impose. I don’t think it’s an accident that even within ISIS, a lot of the Syrian fighters are not involved in certain acts of violence. There are practically no Syrian suicide bombers, and we haven’t seen them involved in the most gruesome acts, like beheadings. Syrians feel uneasy about practices they don’t believe have any basis in the culture or history of their country.

We have heard from Jabhat al-Nusra and Ahrar al-Sham fighters who say ISIS’s foreign fighters have to learn to accept Syrian culture and customs, and while they didn’t say they hate them, it was clear that they did. They said they can only stay [to fight in Syria] if they accept the way that Islam is being practiced in Syria, implying that the foreign fighters practice it differently.

Nusra has been quite clever – in contrast to ISIS – in that wherever they are holding territory, there are always Syrians who are the public face of the organization. So even though they have a number of foreign fighters, those fighters are not on the front line. And that makes Nusra look like a Syrian organization. Meanwhile, we have often heard ISIS referred to by Syrians as “the Foreigners,” because they are perceived to be so predominant in the organization.

Syria Deeply: Are European and British fighters’ roles becoming more visible? Is there a hierarchy within ISIS’s foreign fighter ranks, based on provenance?

Neumann: Europeans are playing a more prominent role in the propaganda because that’s where their particular talents lie. They are playing a more prominent role in the European-language propaganda. The Chechen fighters are extremely influential because they are good fighters, but that’s what they’re out doing – they’re not appearing in the videos.

I haven’t seen any European become very senior. There are Europeans who are in charge of managing other [newer] Europeans, so in that sense there are leaders from Europe. But the problem is that by and large they don’t speak Arabic or have fighting experience, and you can’t be in the upper leadership of a militant Arab organization if you don’t have fighting experience and don’t speak the language of the country that you’re in.

So there are huge obstacles for foreign fighters to be promoted within ISIS. In terms of advancement within the organization, it’s not happening unless you speak the language. The Chechens are seen within ISIS as ideal fighters – ultimate heroes – but the Europeans, not so much. On the other hand, they might be the most committed fighters, and as such they are given more trusted tasks like guarding hostages.

Syria Deeply: How much have the numbers increased in the last two months, and why? What specifically is drawing foreign fighters?

Neumann: We did our last estimate in December, and after that the numbers were stagnant for a long time because of the infighting between different rebel groups – a lot of would-be foreign fighters were deterred by that. They said: “I don’t want to die killing another Sunni rebel.”

Since the caliphate was declared and ISIS emerged as a dominant force, we saw an increase in the number of Westerners going to Syria. I presume the Foley video had impact. Since June, they’ve had so much momentum – and so you’ve seen foreign fighters getting re-energized. For them it’s a good, historical project. They say: “We’re building the caliphate, and this is what people will be talking about in 1,000 years – the brave young Westerners who came from Britain and Holland and France to Syria and Iraq to build the caliphate.” From their point of view, they’re very lucky, to live at this point in history and be a part of this historical project. It’s very exciting to people who are contemplating becoming foreign fighters.

I’d say the number has increased by 10 or 15 percent since July. An extra 200-300 extra people might have gone down. It’s hard to tell because sometimes people talk about going, but then they don’t go. But there has definitely been an increase in intentions.