World leaders have condemned the beheading of US hostage Peter Kassig after it emerged that two British jihadis may have been involved in the brutal killing.
Mr Kassig was captured by Islamic State fighters - also known as IS and Isil - in Syria in October last year while providing medical training and humanitarian aid to victims of the country's conflict.
Footage of his death, broadcast temporarily on YouTube at the weekend, features an IS fighter speaking with a London accent. He is thought to be Jihadi John, the man apparently responsible for previous hostage killings. Another Briton, Nasser Muthana from Cardiff, is also thought to be among the killing squad.
His father, Ahmed, said he was shown a still image from the video which pictured a man who "resembled" his son.
Nasser and his brother Aseel fled the UK in June to fight for IS in Syria. Asked how certain Mr Muthana was that Nasser was among those in the latest video, he said: "Not quite sure. It resembles him. I was shown a picture of the video. I cannot confirm it is him, but I think it might be."
The video also shows what the terrorists claim is a group of at least 10 Syrian pilots being murdered by militants.
Prime Minister David Cameron condemned IS as a "depraved organisation", while US President Barack Obama said the killing of Mr Kassig was "an act of pure evil".
Mr Kassig's parents said their son died "as a result of his love for the Syrian people".
The American aid worker was born Peter but changed his name to Abdul-Rahman following his conversion to Islam while in captivity.
Mr Cameron said: "These beheadings show once again what a depraved organisation this is.
"Like others who have been murdered in cold blood by Isil, Abdul-Rahman Kassig was a selfless humanitarian worker who had gone to the region to help care for those fleeing the Syrian conflict."
Mr Obama praised Mr Kassig as a "humanitarian" who "cared deeply about the plight of the Syrian people".
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. His life and deeds stand in stark contrast to everything that Isil 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 conflict."
Previous beheading videos showed a desert landscape but in this latest release the IS militant appears to be standing in front of a town with fields in the distance. Mr Kassig's severed head is shown at the feet of a militant wearing black trousers and military-style khaki boots.
The latest video is the fifth time this year that IS has published footage showing the murder of Western hostages.
The beheading of James Foley was followed by a video showing the death of fellow US journalist Steven Sotloff, broadcast on September 2.
Footage of the murders of British aid workers David Haines and Alan Henning were made public on September 13 and October 3 respectively.
Mr Kassig's parents, Paula and Ed Kassig, broke their silence over his captivity last month because "the dynamics" had changed and they feared for his life. The couple also released excerpts from a letter written by their son in which he feared "it may very well be coming down to the wire here".
They said in a statement last night: "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. Our heart also goes out to the families of the Syrians who lost their lives, along with our son.
"We know he found his home amongst the Syrian people, and he hurt when they were hurting. We are incredibly proud of our son for living his life according to his humanitarian calling. We will work every day to keep his legacy alive as best we can," they added.