Barack Obama and David Cameron have condemned the on-screen beheading of a journalist amid a hunt for a British jihadist believed to have carried out the murder. The US President said the killing of kidnapped American reporter James Foley by Islamic State (IS) extremists was "hateful", while Mr Cameron branded it "brutal and barbaric".
But both leaders insisted the act would not force them to back away from tackling the group in Iraq and Syria. Mr Cameron cut short his holiday to return to Downing Street and oversee the response after the horrific video was posted online.
Speaking to reporters in Number 10, Mr Cameron said it seemed "increasingly likely" that a British citizen was the killer. "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," he said.
"We have not identified the individual responsible, but from what we have seen it looks increasingly likely that it is a British citizen. This is deeply shocking. But we know that far too many British citizens have travelled to Iraq and travelled to Syria to take part in extremism and violence. And 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 Obama said IS would "fail" because they only wanted to destroy, and the US would "continue to do what we must do to protect our people". "One thing we can all agree on is that Isil has no place in the 21st century," the President said.
"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 who was liked and loved by friends and family."
The film shows Mr Foley, 40, who worked for organisations including Agence France-Presse and went missing in Syria in 2012, 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.
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 IS fighter stood next to him brandishing a knife. Speaking with an apparently English 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 "will result in the bloodshed of your people". Another captured journalist, Steven Sotloff, who went missing near the border of Syria and Turkey last year, is also seen. His life depends on the US president's "next decision", the IS fighter warns. The footage is regarded as authentic by the US and UK, but Scotland Yard has urged people to avoid spreading it through Twitter and Facebook - warning that to do so could be a criminal offence.
Mr Cameron, who is understood to have watched the video himself earlier today, added: "We will stick to the very clear foreign policy and the very clear strategy that we have, which is to work with the new Iraqi government, to help make sure the Kurds get the arms they need to fight off these brutal, extremist militants, to work with our allies and as I've said to use everything we have - our aid, our diplomacy and our military prowess - to make sure that with allies we do everything we can to put the pressure on Islamic States, this appalling organisation, and everything we can back here at home to keep our country safe. That is what we must do."
However, Mr Cameron stressed there would be no "knee jerk" escalation of British military involvement - warning that the West faced a "generational struggle" against Islamist extremism. "I have been very clear that this country is not going to get involved in another Iraq war. We are not putting combat troops, combat boots on the ground, that is not something we should do.
"We have a clear strategy, we should stick to that strategy, and specifically here at home we have very clear laws, tough laws, and of course we will always look at new proposals for even tougher laws to deal with terrorism and extremism. This is not a time for a knee-jerk reaction. It is time for what Britain always shows in these circumstances, and that is a resolve. We have defeated terrorism, extremism, threats to our country before, and we will defeat them again if we show that resolve, but also patience.
"This struggle against Islamist extremism - not a struggle of one religion against another - it is of all people and all religions including Islam against a poisonous extremism, we must show patience and resolve in fighting this, here at home in the UK and in other parts of the world where countries have been affected."
Mr Cameron added: "I have been very clear as Prime Minister over the past four years that this battle that we face against Islamist extremism - not the religion of Islam but a poisonous, extremist, violent narrative - is a generational struggle. It is a battle we have to fight in our own country, it is a battle with allies using everything that we have - our aid, our diplomacy, and yes on occasions our military powers - that we have to fight, whether it is dealing with this problem in Somalia, in Mali, in Afghanistan, in Iraq, in Syria, because as much as we want to focus on keeping ourselves safe here at home, and that is my focus, what happens in these other far-flung places can come back and cause huge harm here too."
In a televised statement from near the presidential holiday home in Martha's Vineyard, Massachusetts, Mr Obama pledged that the US would be "vigilant and relentless" and "do what is necessary to see that justice is done". "Governments and peoples across the Middle East, there has to be a common effort to extract this (IS) cancer so that it does not spread," he said.
"A group like (IS) has no place in the 21st century." Mr Obama said Mr Foley stood for "hope and civility". "A man who lived his work and courageously told the stories of fellow human beings... All of us feel the ache of his absence. All of us mourn his loss."
The killer is referred to as "John" in the footage. A former hostage, who was held for a year in Raqqa, told the Guardian the man was the ringleader of a trio of British born extremists those being held had nicknamed "The Beatles" because of their nationality. They are among several hundred Britons thought to be fighting on the front line in Iraq and Syria.