President Barack Obama confirmed Sunday the death of US aid worker Peter Kassig, calling his beheading at the hands of the Islamic State group "pure evil."

A video released by the militants showed the beheading of Kassig, who took the name Abdul-Rahman after converting to Islam, and that of 18 men described as Syrian military personnel.

The 26-year-old was captured last year and was threatened in an October 3 video showing the beheading of British aid worker Alan Henning.

"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 flew back to the United States from an Asia tour.

"Like 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.

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 in a written statement.

Kassig's mother Paula reached out directly to IS militants to plead for her son's life, even taking to Twitter.

"Please tell us what more we can do so that Abdul-Rahman may continue to serve and live his life in accordance with the teachings of Islam," she wrote in October, in both English and Arabic.

Kerry said that Paula Kassig's "searing plea directed to his captors is unforgettable."

"The fact that her appeal went unheeded is only further testament to the wicked inhumanity of the ISIL terrorists who have taken her son from her," he added.

Kassig founded an aid 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.

"While ISIL revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction, Abdul-Rahman was a humanitarian who worked to save the lives of Syrians injured and dispossessed by the Syrian conflict," Obama said.

"ISIL's actions represent no faith, least of all the Muslim faith which Abdul-Rahman adopted as his own."

He praised Kassig's work as "the selfless acts of an individual who cared deeply about the plight of the Syrian people."

Kassig's parents have asked that he be remembered for his charity work.

"We prefer our son is written about and rememebred for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause."

The former US soldier was the fifth Western hostage killed by IS in recent months, after Sotloff and Foley -- both American journalists -- and two British aid workers were beheaded.

His killing came 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.

Obama expressed confidence 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."

In the undated video, a man who appears to be the same British-accented militant who beheaded previous Western hostages stands above a severed head.

The British government has said it is looking into a newspaper report that the fighter, dubbed "Jihadi John," had been injured in a US-led air strike last week.

Murdered US aid worker Kassig had 'calling' to help Syrians
Washington (AFP) Nov 16, 2014 - Peter Kassig, an American aid worker jihadists claim to have killed, had a "calling" to help Syrians suffering due to the devastating civil war in their country.

The 26-year-old former soldier disappeared on October 1, 2013 and was being held by the Islamic State group, which released a video Sunday claiming Kassig's execution and showing a masked militant standing over a severed head.

Kassig had been in the region since March 2012, when he traveled to Lebanon on a break from university, his family wrote on a Facebook page dedicated to raising awareness about his plight.

After witnessing the plight of those displaced by the war in neighboring Syria, he emailed a letter to family and friends telling them he was not coming home yet.

"I have tried to live my life in a way that displays what it is that I believe, but the truth is, much of my life I have only been searching for my calling, I had not yet found it," he wrote.

"Here, in this land, I have found my calling."

The young American, who was also trained as an emergency medical technician, went to Turkey and founded an aid group, his father, Ed Kassig, said in a video last month.

"He helped train 150 civilians in skills needed to provide medical aid to the people of Syria. His organization gave food, cooking supplies, clothing and medicine to those in need," Ed Kassig said as he pleaded for his son's release.

- Desolation and hope -

In an email to a friend in 2012, also posted on the Facebook page, Kassig wrote about the desolation of war and his hope for the future.

"How will I tell you of the walls riddled with bullets and the flowers that grow over them?" he wrote.

"War never ends, it just moves around... Loss and destruction in this land brings about only survival; the determination to press on and rebuild," he added.

"I can't be sure but I think I'm starting believe that as beautiful as it is to finish building a house, its a better feeling laying the first brick again after its been torn down..."

In a quotation on the Facebook page, he admits he at times wished for something different. The quote is attached as a caption to a photo showing Kassig brandishing a blood-clotting agent he obtained to help treat wounded in Syria

"The truth is sometimes I really think I would like to do something else, but at the end of the day, this work is really the only thing that I have found that gives my life both meaning and direction."

Kassig, who converted to Islam during his captivity and took the Muslim name Abdul-Rahman, said in a letter smuggled to his family his faith was intact.

"In terms of my faith, I pray every day, and I am not angry about my situation in that sense. I am in a dogmatically complicated situation here, but I am at peace with my belief."

- 'Scared to die' -

In an October 3 video showing British aid worker Alan Henning's beheading, the threat was made that Kassig would be next.

IS has justified killing Western hostages as retaliation for US-led air strikes on the group over swathes of territory it has seized in Iraq and Syria.

In the letter received by his family in June, Kassig wrote that he was "scared to die".

"But the hardest part is not knowing, wondering, hoping and wondering if I should even hope at all. I am very sad all this has happened and for what all of you back home are going through.

"If I do die, I figure that at least you and I can seek refuge and comfort in knowing that I went out as a result of trying to alleviate suffering and helping those in need."