ABOARD AIR FORCE ONE — The White House on Sunday confirmed the death of U.S. aid worker Peter Kassig, a former soldier who tried to help wounded Syrians caught in a brutal civil war but ended up dying himself at the hands of Islamic State militants.

President Barack Obama, in a statement issued as he flew back to Washington after a trip to the Asia Pacific region, said the group “revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction.”

With Kassig’s death, IS has killed five Westerners it was holding. Britons David Haines, a former Air Force engineer, and Alan Henning, a taxi driver from northwest England, were beheaded, as was U.S. reporter James Foley and American-Israeli journalist Steven Sotloff.

The U.S. confirmation about Kassig came after a review of an IS video released Sunday that purported to show extremists beheading a dozen Syrian soldiers .