More than 25,000 foreign fighters have travelled to join militant groups such as al-Qaeda and Islamic State (IS), according to a UN report.
Experts have said that the flow of foreign fighters is "higher than it has ever been historically".
Fighters from at least 100 countries have travelled to areas such as Iraq, Syria, Libya and Pakistan.
The report states that this poses an immediate and long-term threat to global security.
It said that the number of foreign fighters has "risen sharply", increasing by 71% between the middle of 2014 and March 2015.
Syria and Iraq have also become a "veritable finishing school for extremists", with some 22,000 foreign fighters.