Barack Obama has described the beheading of US aid worker Peter Kassig by Islamic State militants as an "act of pure evil".
The President confirmed the authenticity of a video posted online showing Mr Kassig's severed head.
It is not known when the footage, which features a hooded militant speaking in a British accent, was shot.
Mr Obama offered his "prayers and condolences" to the family of Mr Kassig, who is also known as Abdul-Rahman - the name he took following his conversion to Islam while in captivity.
In a statement the President said: "Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity.
"Like Jim Foley and Steve Sotloff before him, his life and deeds stand in stark contrast to everything that ISIL (IS) represents.
"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."
In a statement Kassig's parents, Ed and Paula Kassig, said their son lost his life trying to ease the suffering of the Syrian people.
"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," the statement said.
"Our heart also goes out to the families of the Syrians who lost their lives, along with our son.
"Fed by a strong desire to use his life to save the lives of others, Abdul-Rahman was drawn to the camps that are filled with displaced families and to understaffed hospitals inside Syria.
"We know he found his home amongst the Syrian people, and he hurt when they were hurting."
In the video, a militant stands before the severed head and says: "This is Peter Edward Kassig, a US citizen, of your country; Peter who fought against the Muslims in Iraq, while serving as a soldier.
"We say to you Obama, you claim to have withdrawn from Iraq four years ago. We said to you then that you were liars, that you had not withdrawn and that if you had withdrawn, that you would return."
The militant issues threats to Britain and the US, adding: "Obama, dog of Rome, today we are slaughtering the soldiers of Bashar (al Assad, President of Syria)."
The video shows what is apparently the mass beheading of captured Syrian soldiers.
The fighter appears to be the man known as 'Jihadi John' - who is believed to be responsible for previous hostage killings and who was reportedly injured in a US airstrike.
Prime Minister David Cameron condemned the killing, saying: "This underlines the deplorable depths to which these terrorists are prepared to go - savagely murdering a compassionate man."
Previous videos have shown a desert landscape, but in the latest footage the militant seems to be in front of a town with fields in the distance.
The footage identifies the militant's location as Dabiq, a small town in Syria's Aleppo province, near the border with Turkey.
Mr Kassig, a 26-year-old from Indiana, was taken hostage in October 2013.
His beheading is the fifth such killing of a Westerner by IS, following the deaths of US journalists Steven Sotloff and James Foley and British aid workers David Haines and Alan Henning.
