At least 5,600 supporters of so-called Islamic State (IS) have returned to their home countries as it loses ground in Iraq and Syria, a new report says.

The Soufan Center, a US-based think tank, says 33 states have reported arrivals in the past two years.

The figure includes half of the estimated 850 people who left the UK.

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.