(BBC) - US President Barack Obama has condemned the killing of US aid worker Abdul-Rahman Kassig as "an act of pure evil".

The Islamic State (IS) militant group released a video showing a masked man standing over a severed head, which the White House confirmed was Mr Kassig.

His parents said he died "as a result of his love for the Syrian people".

Mr Kassig, 26, was taken while working for a refugee group. He is the fifth Western hostage to be killed by IS, which controls parts of Syria and Iraq.

The president praised Mr Kassig as a humanitarian and said he was "taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity".

"Today we grieve together, yet we also recall that the indomitable spirit of goodness and perseverance that burned so brightly in Abdul-Rahman Kassig," he said.

Mr Obama's comments came as he flew back to the US from the G20 summit in Australia.

Mr Kassig's parents, Ed and Paula, from Indiana, said in a statement they were heartbroken by his death.

"We are incredibly proud of our son for living his life according to his humanitarian calling," they said.

"We will work every day to keep his legacy alive as best we can."

'British' jihadist

The other Western hostages killed by IS were Britons Alan Henning and David Haines, and US journalists James Foley and Steven Sotloff.

Those killings were carried out by a man believed to be British. That man bears a resemblance to one of the masked militants pictured in the video showing Mr Kassig.

Unlike previous videos released by IS, the latest one shows the faces of many of the jihadis and specifies its location - Dabiq in Syria's Aleppo province.

It also shows the beheading of 18 Syrian captives, who are identified as army officers and pilots.