A new report says Islamist groups such as the Islamic State have more than doubled their recruitment of foreign fighters to least 27,000 in the past 18 months.
The report from the Soufan Group, a New York-based intelligence consultancy, says Arab countries supply the greatest number of recruits.
It says recruitment has risen markedly in Western Europe and the former Soviet Union, particularly Russia's North Caucasus republics of Chechnya and Daghestan and Central Asian countries.
The authors say the largest single group of foreign fighters is Tunisians, numbering some 6,000.
The second-largest is from Saudi Arabia (2,500), followed by Russia (2,400). Some 5,000 fighters come from EU countries.
The document says the average rate of returnees to Western countries is now at up to 30 percent.
This presents "a significant challenge to security and law-enforcement agencies that must assess the threat they pose," it says.
Based on reporting by Reuters and the BBC