BEIRUT - 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.
The video also showed the gruesome simultaneous beheadings of at least 18 men described as Syrian military personnel, the latest in a series of mass executions and other atrocities carried out by IS in Syria and Iraq.
"Like (US journalists) Jim Foley and Steven Sotloff before him, his life and deeds stand in stark contrast to everything that ISIL represents," Obama added, using another acronym by which IS is known.
"ISIL's actions represent no faith, least of all the Muslim faith that Abdul-Rahman adopted as his own." In the undated video, a man who appears to be the same British-accented jihadist who beheaded previous Western hostages stands above a severed head.
"Here we are burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive," the black-clad masked executioner says, referring to a northern Syrian town.
He also urges Obama to send more troops back to the region to confront IS, just as Washington prepares to double its military personnel in Iraq to up to 3,100 as part of the international campaign it heads against IS.
Kassig, a former US soldier, had risked his life to provide medical treatment and aid to those suffering from Syria's civil war.
He founded a group through which he trained some 150 civilians to provide medical aid to people in Syria. His group also gave food, cooking supplies, clothing and medicine to the needy.
"We are incredibly proud of our son for living his life according to his humanitarian calling," his parents wrote, using the Twitter handle #kassigfamily.
US Secretary of State John Kerry said that American government officials worked alongside Kassig's family to try to secure his release.
"During his time in captivity, his family, and the entire government, including his home state Senator Joe Donnelly, worked to avoid this tragic outcome," the top US diplomat said.
Kassig was the fifth Western hostage killed by IS in recent months, after the two US reporters and two British aid workers were beheaded.
Sunday's video was substantially different from previous IS recordings. Kassig was not shown alive in the footage, and no direct threats were made against other Western hostages.
British Prime Minister David Cameron said he was "horrified" by the "cold-blooded murder," which French President Francois Hollande called a "crime against humanity." Burhan Mousa Agha, a Syrian friend who worked with Kassig in Lebanon, described him as a funny, dedicated and brave man who only wanted to help.
"I want to apologise to his family. I'm sorry that their son died in my country, trying to help," Agha told AFP.
"They are animals, less than animals, they don't represent Islam. Peter wasn't fighting anyone, he was teaching people how to save lives.
"I want to send a message to everyone that Peter was a hero, a real hero," Agha added.