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

"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 aboard Air Force One as he flew back to the US from an Asia tour. 

The self-styled Islamic State jihadist group had earlier claimed to have executed Mr Kassig, a US aid worker kidnapped in Syria, as a warning to the United States.

The same video 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.

Mr Kassig was previously known as Peter, but converted to Islam while in captivity and adopted the name Abdul Rahman, a family spokesperson said last month.

A militant wearing a balaclava appeared in footage saying: "This is Peter Edward Kassig, a US citizen of your country."

He was wearing the same outfit worn by the man who beheaded two American journalists and two British aid workers in earlier videos.

The man stood over a severed head bearing a resemblance to Mr Kassig, a former American soldier who risked his life to provide medical treatment and aid to those suffering from Syria's civil war.

"Here, we are burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive," the militant said.

Dabiq is the site of a major 16th century battle in what is now northern Syria that saw the Ottomans defeat the Mamluks and begin a major expansionist phase of an empire the IS group considers to have been the last caliphate.

In a highly choreographed sequence earlier in the video, jihadists marched at least 18 prisoners said to be Syrian officers and pilots by a wooden box of long military knives, each taking one as they passed, then forced them to kneel in a line and decapitated them.

IS spearheaded a militant offensive that overran much of Iraq's Sunni Arab heartland since June after seizing major territory in neighbouring Syria, and carried out a series of atrocities in both countries.

The group has killed hundreds of Iraqi and Syrian tribesmen who opposed it, attacked religious and ethnic minorities, sold women as slaves, executed scores of Iraqi security personnel and carried out beheadings on camera.

In a letter to his parents last month, Mr Kassig wrote:

"The first thing I want to say is thank you. Both to you and Mom for everything you have both done for me as parents; for everything you have taught me, shown me, and experienced with me.

"I cannot imagine the strength and commitment it has taken to raise a son like me but your love and patience are things I am so deeply grateful for.

"I am obviously pretty 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 that 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.'

"I wish this paper would go on forever and never run out and I could just keep talking to you.

"Just know I'm with you. Every stream, every lake, every field and river. In the woods and in the hills, in all the places you showed me. I love you."

Killings condemned

French Prime Minister Manuel Valls said he "condemns this new barbaric act. This deed reinforces France's determination to act against Daesh in Iraq and Syria," in a statement using another name for the IS group.