Abdul-Rahman Kassig killing is pure evil, says Obama
US President Barack Obama has condemned the killing of US aid worker Abdul-Rahman Kassig as "an act of pure evil".
The Islamic State (IS) militant group released a video showing a masked man standing over a severed head, which the White House confirmed was Mr Kassig's.
His parents said he died "as a result of his love for the Syrian people".
Mr Kassig, 26, was taken while working for a refugee group. He is the fifth Western hostage to be killed by IS, which controls parts of Syria and Iraq.
Analysis: Frank Gardner, BBC security correspondent
This is perhaps the most elaborate and graphic of all the murder videos posted online by IS. Much of it is taken up with a recent history lesson on Iraq and Syria as seen through the eyes of the jihadists. But the latter part shows the beheading of 18 Syrian prisoners in revolting, lingering detail.
Unlike earlier videos, this one revels in gore. Amongst the boiler-suited captives murdered in cold blood is a man IS says is the former US soldier Peter Kassig, who converted to Islam and changed his name to Abdul-Rahman.
Neither his conversion, nor the fact that he was helping refugees when he was captured a year ago, appear to have saved him.
Mr Kassig's murder is a sign of frustration that IS militants are unable to hit back at the coalition air strikes that have driven them off key sites like the Mosul and Haditha dams, and prevented them from seizing the town of Kobane.
The US president praised Mr Kassig as a humanitarian and said he had been killed "in an act of pure evil by a terrorist group that the world rightly associates with inhumanity".
He paid tribute to the dead man's "indomitable spirit of goodness and perseverance".
Mr Obama's comments came as he flew back to the US from the G20 summit in Australia.
Mr Kassig's parents, Ed and Paula, from Indiana, said in a statement they were heartbroken.
"We are incredibly proud of our son for living his life according to his humanitarian calling," they said.
"We will work every day to keep his legacy alive as best we can."
- Former US Army Ranger who served in Iraq in 2007
- Trained as an emergency medical technician and founded an organisation to supply refugee camps in Syria
- Snatched in October 2013 while travelling in eastern Syria for a project
- Changed his name from Peter Kassig when he converted to Islam in late 2013
The other Western hostages killed by IS were Britons Alan Henning and David Haines, and US journalists James Foley and Steven Sotloff.
Those killings were carried out by a man believed to be British. That man bears a resemblance to one of the masked militants pictured in the video showing Mr Kassig.
Unlike previous videos released by IS, the latest one shows the faces of many of the jihadis and specifies its location - Dabiq in Syria's Aleppo province.
It also shows the beheading of 18 Syrian captives, who are identified as army officers and pilots.