President Barack Obama has confirmed the execution of U.S. aid worker Peter Kassig by Islamic State terrorists.

Kassig, who had converted to Islam and changed his name to Abdul-Rahman Kassig, "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.

Islamic State had posted a video online showing the severed head of Kassig, who's the sixth foreign captive executed by the militants since they took control of parts of Iraq and Syria earlier this year. It was similar to videos that showed the executions of U.S. journalists James Foley and Steven Sotloff.

"ISIL's actions represent no faith, least of all the Muslim faith which Abdul-Rahman adopted as his own," the president said, using another name for Islamic State.

"Today we grieve together, yet we also recall that the indomitable spirit of goodness and perseverance that burned so brightly in Abdul-Rahman Kassig, and which binds humanity together, ultimately is the light that will prevail over the darkness of ISIL," Obama said.

A U.S.-led coalition has targeted Islamic State with hundreds of airstrikes.