The number of foreign fighters in Iraq and Syria has more than doubled in 18 months, according to a recent study by the Soufan Group.

The report estimates that between 27,000 and 31,000 people from at least 86 countries have traveled to Syria and Iraq to join ISIL and other extremist groups. Out of the 5,000 foreign fighters from Western Europe, 3,700 came from just four countries: France, Germany, Belgium, and the United Kingdom.

When researchers carried out the same study in the summer of 2014, the number of foreign fighters was estimated to be around 12,000. International efforts from world leaders to stem the number of their citizens going to fight for ISIL appears to have little effect.

The number of fighters going to Iraq and Syria from North America has remained “relatively flat,” the report notes. But, there has been a substantial jump in the number of foreign fighters from Russia and Central Asia, the report estimates a near 300% increase since June 2014.

While ISIL’s slick online propaganda has played an important role in recruiting people in the US, researchers point out that in the countries with the largest flows of foreign fighters, face-to-face recruitment has played a greater role. In these countries, recruitment has been far more localized, focused, and more successful.

Researchers also analyzed neighborhoods with the highest concentration of foreign fighters, so-called “hotbeds of recruitment.” The Molenbeek suburb in Brussels was under the spotlight last month when a raid led to seven arrests in the neighborhood after several of the Paris attackers came from the area.

Molenbeek has been embroiled in a range of terrorist incidents, including the 2004 Madrid bombings and last year’s Jewish Museum shooting in Brussels. The report describes Molenbeek as a relative “newcomer to the production of foreign fighters,” and emphasizes the role of two Tunisian towns—Bizerte and Ben Gardane—and Derna in Libya, which have a longer history of sending fighters to join extremist groups.