US President Barack Obama has said the beheading of aid worker Peter Kassig by Islamic State (IS) was "an act of pure evil".

A video was released on Sunday showing a masked militant standing over the severed head of a man it said was the former US Army Ranger, who was seized while delivering relief supplies in Syria last year.

Mr Obama confirmed Mr Kassig's death after a US review of the video, which also showed the mass beheading of a dozen Syrian soldiers.

The 26-year-old, who founded an aid group to help Syrians caught in their country's brutal civil war, "was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity", Mr Obama said.

He denounced the ­extremist group, which he said "revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction".

The dead hostage's parents, Ed and Paula Kassig, said they were "heartbroken" by their son's killing but "incredibly proud" of his humanitarian work.

They both attended a vigil held in Mr Kassig's home city of Indianapolis in the state of Indiana.