More than 25,000 foreign fighters from 100 nations have travelled to join militant groups such as al-Qaeda and Islamic State (IS), a UN report says.
It said the number of foreign fighters worldwide had soared by 71% between the middle of 2014 and March 2015.
Syria and Iraq were by far the biggest destinations and had become a "finishing school for extremists".
It also said if IS were defeated in Syria and Iraq, the foreign fighters could be scattered across the world.