The Number of Foreign Fighters Who Travelled to Syria and Iraq Doubled Since Last Year

As many as 31,000 people from at least 86 countries have travelled to Syria and Iraq to fight for the Islamic State and other extremist groups since 2011, according to researchers.

In a new report, the Soufan group, a New York based extremism tracking consultancy, said those figures had roughly doubled since an earlier assessment made in the summer of 2014. Though the latest numbers largely confirm earlier estimates made by the US and the United Nations, the data provides more granular accounts of countries and regions from which thousands of fighters have emerged.

About half of the foreign fighters hailed from the Middle East and North Africa. Militants from Tunisia, who may number as many as 7,000, continue to have an outsized presence in Jihadi groups in Iraq and Syria. In a country of less than 11 million people, such a figure is a huge concern for officials in Tunis; a similar proportion of Americans would amount to over 200,000 people.

In Europe, 3,700 of an estimated 5,000 plus fighters have travelled from just four countries: France, Belgium, Germany and the United Kingdom. French official say that some 1,800 have emerged from within their borders including several who reportedly returned to carry out terror attacks in Paris that left 131 dead in November.

"The increase is evidence that efforts to contain the flow of foreign recruits to extremists groups in Syria and Iraq have had limited impact," wrote the report's authors.

Related: Two Suspected Foreign Fighters Among the Dead in Latest al Shabaab Attack in Kenya

But by other measures some progress has been made.

In an interview with VICE News, Patrick Skinner, director of special projects at the Soufan Group said the influx likely peaked starting at the end of last year, following ISIL's blitzkrieg expansion in parts of northern Iraq. "It exploded in the fall and winter," said Skinner.

Moreover, due to limited national reporting, Soufan's tabulations, like a 30,000 figure arrived at by US intelligence officials, would be expected to include fighters who were killed or returned home. In June of this year, State Department officials estimated 10,000 IS fighters, both local and foreign, had been killed since the start of US-led airstrikes in 2014. That number is doubtlessly higher today, particularly after stepped up involvement from countries like France in anti-IS airstrikes.

"They have lost a lot of people," said Skinner.

Skinner said a large portion of those who travelled to Iraq and Syria — between 20 and 30 percent — have also retraced their steps, and emerged in their home countries, particularly in Europe. While that means there could be fewer foreign extremists in both countries than data indicates, it presents a worrying specter, particularly if they were sent back on missions rather than having given up on fighting.

According to the Soufan Group's latest estimates, some 2,500 fighters from Saudi Arabia, and 4,700 from the former Soviet Union, have emerged in Iraq and Syria since 2011.

Related: Amnesty for Repentance: Tajikistan's Radical Approach on Returning Islamic State Fighters

Compared to some of its Western and Arab allies, the US has seen a far smaller proportion of its residents travel to Iraq and Syria. According to intelligence estimates, more than 250 have attempted to or succeeded in getting to one of the two countries. A September study issued by the House Homeland Security Committee found that out of 58 individual cases studies, 12 American fighters had been killed after joining jihadists groups  — roughly 20 percent. The same study reported that around 40 Americans have already returned from Syria, but the committee found that only "five of them had been arrested by authorities."

The Soufan report said that social media plays an outsized role in recruiting Americans, who lived thousands of miles farther from IS-controlled territory than most who join the group's ranks. But the report added that even the power of the internet to radicalize young men may be overblown.

"While the power of the Islamic State's social media outreach is undeniable, it appears more often to prepare the ground for persuasion, rather than to force the decision," said the report.

Watch VICE News' documentary The Battle for Iraq: Shia Militias vs. the Islamic State: