US President Barack Obama condemned as "pure evil" the Islamic State's beheading of American aid worker Peter Kassig after the group released a video showing his body Sunday.

Kassig's parents said they were "heartbroken" by their loss after IS made public the recording in a warning to Washington as it prepares to send more troops to Iraq.

"We are heartbroken to learn that our son, Abdul-Rahman Peter Kassig, has lost his life as a result of his love for the Syrian people and his desire to ease their suffering," Ed and Paula Kassig said in a statement posted on Twitter.

Kassig, who took the name Abdul-Rahman after converting to Islam, was captured last year and was threatened in an October 3 video showing the beheading of British aid worker Alan Henning. He was 26 years old.

"Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity," Obama said in a statement released by the White House aboard Air Force One as he returned home from an Asia tour.