Peter Kassig beheading video labelled act of 'pure evil' by Obama

US leader offered his 'prayers and condolences' for family of aid worker Peter Kassig who was killed by extremist group

President Obama confirmed the death of the third American at the hands of Isis (Reuters)

President Barack Obama has condemned the murderers of American aid worker Abdul-Rahman Kassig, labelling the killing “pure evil”.

The US president confirmed the death of the humanitarian worker, who changed his name from Peter when he converted to Islam, yesterday. Reports indicate that Mr Kassig, 26, was killed by the militant Islamist group Isis, also known as Islamic State, on Friday.

“Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world right associates with inhumanity,” Mr Obama said in the statement, expressing his “prayers and condolences” to Mr Kassig’s family.

Isis, the statement reads, “revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction.”

Mr Obama makes a point of referring to Mr Kassig as Abdul-Rahman, the name he adopted after his conversion to Islam, rather than his Christian name of Peter, in the statement read on board Air Force One yesterday.

Mr Kassig "lost his life as a result of his love for the Syrian people and his desire to ease their suffering," his parents said in a statement from Indianapolis. It is believed he was captured by Isis in October.

A former US Army Ranger, Mr Kassig was honourably discharged after 15-months service – including a stint in Iraq – and had retrained as a medical emergency technician before founding an aid group to help Syrians caught up in their nation’s brutal civil conflict.

The US aid worker Peter Kassig in Syria, where he was held hostage in 2013 (EPA)

In the video, released early Sunday morning, an Isis militant believed to be the man known as ‘Jihadi John’ says: “To Obama, the dog of Rome. Today we are slaughtering the soldiers of Bashar [al-Assad, the Syrian leader] and tomorrow we will be slaughtering your soldiers.”

'Jihadi John', whose British accent has been distorted in an attempt to conceal his identity, then continues: “You claim to have withdrawn from Iraq four years ago. We said to you then that you are liars.”

Prime Minister David Cameron has also condemned the “depravity” of the militants, praising Mr Kassig as a “selfless humanitarian”.

The footage released yesterday identifies the militants' location as Dabiq, a town in northern Syria that Isis uses as the title of its English-language propaganda magazine and where they believe an apocalyptic battle between Muslims and their enemies will occur.

The high-definition video also showed the beheadings of about a dozen men identified as Syrian military officers and pilots, all dressed in blue jumpsuits. The black-clad militant warns that US soldiers will meet a similar fate.

Mr Kassig is the fifth westerner to be killed by the extremist group. His death, six weeks after the murder of British aid worker Alan Henning. Americans James Foley and Steven Sotloff and British man David Haines have also been killed by Isis.

British journalist John Cantlie is still being held by the terror organisation.