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.
The report, by experts monitoring UN sanctions against al-Qaeda, warned that defeating IS in Iraq and Syria could lead to the dispersal of experienced fighters across the world.
There are also 6,500 fighters in Afghanistan and hundreds in Yemen, Libya and Pakistan, the report says.
It adds that a high number of foreign fighters have come from Tunisia, Morocco, France and Russia.
There has also been an increase in the number coming from the Maldives, Finland and Trinidad and Tobago.
The report called for greater intelligence sharing between nations to help identify foreign fighters.