On Friday, the Hague-based International Center for Counterterrorism (ICCT) reported that most EU inhabitants who have left to fight in Syria and Iraq come from four countries.

According to the ICCT, 2,838 (76.5 percent) of the 3,710 Europeans fighting alongside militant groups in the Middle East are citizens or residents of Germany, France, the UK and Belgium.

"Many (member states) consider returning foreign fighters as a potential security threat; this research indicates that an average of 30 percent of foreign fighters have returned to their countries of departure," the report found.

The study, commissioned for the Netherlands' EU presidency, confirmed previous reports that Belgium had the highest per capita number of foreign fighters, while France provided the highest real number of citizens or residents joining conflicts abroad, with over 900.

Germany tied with the UK in terms of real numbers of foreign fighters, with between 720 and 760 leaving the country to fight abroad, of whom 250 have repatriated.

Denmark, the UK and Germany accounted for the highest percentages of returnees, with 50 percent, 48 percent and 33 percent, respectively.

Motivation: From Assad to the caliphate

The report found that there were several motivating factors that have contributed to EU residents' joining armed conflicts in Iraq and Syria.

The center named some of these as "solidarity with other 'fellow Muslims' abroad (in Syria mostly, and especially during the early stages of foreign fighter travel), the fight against the Alawite Assad regime in Syria, the desire to live in a territory ruled by Islamic law, alienation and social exclusion felt in Europe, as well as the desire to conduct jihad."

"For some, the search for excitement and adventure play a role, as does peer pressure and the prospects of life in the caliphate, such as marriage and housing," the report added.

Who is a foreign fighter?

In the questionnaire provided to member states, the center inquired about the groups that EU nationals or residents have joined, making distinctions between the "Islamic State" (IS), US-allied Kurdish militias, Syrian pro-government forces and opposition groups, such as the Free Syrian Army.

Though the ICCT included groups that were blacklisted by the EU, "it became clear that in many responses, member states implicitly or explicitly focused only on foreign fighters that joined the Islamic State in Syria and, more marginally, Iraq."

"On the other hand, a small number of countries also included individuals who participated in other conflicts, such as Mali, Somalia, Ukraine and Libya," the report added.