At least 5,600 supporters of so-called Islamic State (IS, ISIS, Daesh) have returned to their home countries as it loses ground in Iraq and Syria, BBC reported.
The Soufan Center, a US-based think tank, says 33 states have reported arrivals in the past two years.
The report says the returnees - most of whom are imprisoned or disappear from view - will continue to present a security challenge for years to come.
