A video showing an Islamic State (IS) extremist with a British accent beheading an American journalist appears to be genuine and is an "appalling example of the brutality of this organisation", Foreign Secretary Philip Hammond has said.
In the video James Foley, who went missing in Syria in 2012, is seen kneeling in a desert-like environment at an unknown location as an IS fighter stands by his side dressed in black and with his face covered.
Hammond said the video had not been verified but "all the hallmarks point to it being genuine" and acknowledged that the killer spoke with a British accent.
The Foreign Secretary told the BBC: "Certainly at first pass that's what it looks like and we will obviously want to investigate that further.
"We have been saying for a long time that there are a significant number of British nationals in Syria and Iraq operating with extremist organisations.
"That's one of the reasons why this organisation represents such a direct threat to the UK's national security.
"Many of these people may seek at some point to return to the UK and they would then pose a direct threat to our domestic security."
Hammond said the intelligence agencies were tracking and monitoring Britons who could be involved with extremist groups in Syria and Iraq.
He acknowledged that "significant numbers" of British nationals had been involved in "terrible crimes" and "probably in the commission of atrocities".
He said: "We are very much aware of the threat from foreign nationals operating in Syria and Iraq, that's not just to the UK it's to all Western countries and other Arab countries as well.
"This is a poison, a cancer, what's going on in Iraq and Syria and it risks spreading to other parts of the international community and affecting us all directly."
He added: "This may come as news to some people but it certainly does not come as news to us.
"We are absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with IS and other extremist organisations.
"This is something we have been tracking and dealing with for many, many months, I don't think this video changes anything it just heightens awareness of a situation which is very grave and which we have been working on for many months."
In the video Foley, wearing an orange outfit similar to those worn by Guantanamo Bay inmates, makes a statement during the five-minute video entitled "A message to America" released by the Islamic State Al-Furqan foundation.
During the speech he said he wanted to call on family, friends and loved ones to "rise up against my real killers, the US government".
He added: "For what will happen to me is only the result of their complacency and criminality".
The statement, during which he is seen to pause and take deep breaths, ended with him saying he wished he could see his family again.
The IS fighter standing next to him is then seen brandishing a knife before accusing America of "aggression towards the Islamic state".
He said: "You have plotted against us and gone far out of your way to find reasons to interfere in our affairs.
In a chilling threat in which he singles out US president Barack Obama, he says further actions in Iraq by America "will result in the bloodshed of your people".
After the victim is apparently beheaded, another man, said to be American journalist Steven Sotloff, who went missing near the border of Syria and Turkey last year, is shown at the end of the video, with the IS fighter saying his life depends on President Obama's "next decision".
Foley's mother Diane released a statement saying the family "have never been prouder" of him, and pleading with the kidnappers not to kill any other hostages.
"We have never been prouder of our son Jim. He gave his life trying to expose the world to the suffering of the Syrian people.
"We implore the kidnappers to spare the lives of the remaining hostages.
Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world.
"We thank Jim for all the joy he gave us. He was an extraordinary son, brother, journalist and person. Please respect our privacy in the days ahead as we mourn and cherish Jim."
Obama is expected to make a statement on the killing later today.
Foley was on an assignment for Agence France-Press and the Boston-based media company GlobalPost in northern Syria when his car was stopped by militants in 2012.
The 40-year-old freelancer from Rochester, New Hampshire, has not been heard from since. He is one of an estimated 20 journalists missing in Syria, according to the New York-based Committee to Protect Journalists.
He was previously held captive by the government in Libya alongside a small group of other journalists, but was released and given a one-year suspended sentence on charges of illegally entering the country.
In an interview with the Associated Press about that experience he said he "would love to go back", but recalled the tragedy of seeing a colleague killed in a firefight.
He also spoke of a love of his profession.
"Journalism is journalism," he said. "If I had a choice to do Nashua (New Hampshire) zoning meetings or give up journalism, I'll do it. I love writing and reporting."
He told the BBC in a 2012 interview that he was "drawn to the drama of the conflict and trying to expose untold stories".
He said: "There's extreme violence, but there's a will to find who these people really are. And I think that's what's really inspiring about it."
Philip Balboni, chief executive of GlobalPost paid tribute to their employee, and thanked the public on behalf of Mr Foley's parents.
"On behalf of John and Diane Foley, and also GlobalPost, we deeply appreciate all of the messages of sympathy and support that have poured in since the news of Jim's possible execution first broke.
"We have been informed that the FBI is in the process of evaluating the video posted by the Islamic State to determine if it is authentic. … We ask for your prayers for Jim and his family."