They are the Islamic State group's elite shock troops. Fanatical and disciplined, they infiltrate cities of Iraq and Syria, unleash mayhem and fight to the death, wearing explosives belts to blow themselves up among their opponents if they face defeat.

IS calls them "Inghemasiyoun", Arabic for "those who immerse themselves", a sort of special forces unit parallel to its regular forces that is credited with many of the group's stunning battlefield successes.

"They cause chaos and then their main ground offensive begins," said Redur Khalil, spokesman for the US-backed Kurdish People's Protection Units, which have led a string of military successes against the IS in Syria.