UK and US intelligence services are engaged in an international manhunt to find the identity of the British jihadist who beheaded the American journalist James Foley.
Reports suggest a former hostage claims the Islamic State (IS) terrorist pictured in the footage of Mr Foley's murder and who called himself John, was the ringleader of three British jihadists, who guarded foreign nationals in Raqqa in Syria, a rebel stronghold. They have been dubbed "the Beatles" by hostages because of their nationality.
David Cameron, who broke off from his holiday in Cornwall after a day to return to Downing Street to chair an emergency meeting of senior ministers and officials, watched the footage of the 40-year-old reporter's killing.
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The Prime Minister said: "Let me condemn the barbaric and brutal act that has taken place and let's be clear what this act is: it is an act of murder and murder without any justification.
"We have not identified the individual responsible but, from what we have seen, it looks increasingly likely that it is a British citizen."
President Barack Obama, who remained on holiday in Martha's Vineyard, Massachusetts, insisted in a televised statement the US Government would do all it could to protect American citizens.
"We will continue to confront this hateful terrorism and replace it with a sense of hope and stability. That's what Jim Foley stood for; a man, who lived his work, who courageously told the stories of his fellow human beings and was loved by friends and family."
The IS film shows Mr Foley, who went missing in Syria in 2012, kneeling in a desert-like environment at an unknown location as a terrorist stands by his side dressed in black and with his face covered. Pausing and taking deep breaths, the distraught journalist said he wanted to call on family, friends and loved ones to "rise up against my real killers - the US Government" as the terrorist stood next to him brandishing a knife.
Speaking with an apparent London accent, the killer accused America of aggression towards the Islamic state, adding: "You have plotted against us and gone far out of your way to find reasons to interfere in our affairs." He said further actions in Iraq by America would "result in the bloodshed of your people".
There are thought to be up to 20 journalists of different nationalities being held by IS. It is estimated that 500 Britons have gone to Syria to take part in the fighting. One unconfirmed suggestion is that a further 250 have already returned to the UK. They include Abdul Rakib Amin, thought to be 25, from Aberdeen, who appeared in an IS recruitment video.
Mr Cameron, who has acknowledged that the return of radicalised Britons to the UK is the country's biggest security threat, added: "We know that far too many British citizens have travelled to Iraq and travelled to Syria to take part in extremism and violence. What we must do is redouble all our efforts to stop people from going, to take away the passports of those contemplating travel, arrest and prosecute those who take part in this extremism and violence, to take extremist material off the internet and do everything we can to keep our people safe. And that is what this Government will do."
Mr Foley's parents, Diane and John Foley, spoke to reporters outside their home in Rochester, New Hampshire. Mrs Foley said her son was courageous to the end and called his death "just evil".
In a statement on a webpage, she added that her son "gave his life trying to expose the world to the suffering of the Syrian people". Asked in January 2013 if her son had reservations about going to Syria, Mrs Foley had said softly: "Not enough."
She wrote on the family's webpage: "We thank Jim for all the joy he gave us."