James Foley 'beheaded': Isis video shows militant with British accent 'execute US journalist' – as hunt begins for killer

David Cameron is holding crisis talks in Downing Street to lead the Government's response to a the murder of a US journalist by an Islamic State (Isis) militant thought to be from Britain.

The Prime Minister cut short his holiday after a video was released overnight showing what appears to be a British man beheading an American reporter, James Foley, in what he claims is a revenge killing for the ongoing US air strikes in Iraq.

Mr Cameron will be briefed by senior intelligence officials as intelligence services try to piece together evidence that could identify the killer.

The killing has been condemned internationally, with France branding it a "cowardly assassination" and White House officials saying the US is "appalled".

Earlier today, the Foreign Secretary, Philip Hammond, confirmed that the militant "appeared to be British" condemned the killing as “one more example in a catalogue of brutality” by "evil" Isis.

Mr Hammond said Isis was not just a threat to Syria and Iraq, where it has killed countless civilians and caused thousands of people to flee in terror, but to our international security.

“If Isis becomes established in Iraq and Syria it will undoubtedly use it as a base for launching attacks on the West,” he told Radio 4’s Today programme. "It will undoubtedly send its fighters out to attack Western targets.

“Equally, if it gets pushed back, some of these people will return to their country of origin and it’s not just the UK.

“We will see these people going back and potentially carrying on their fight in our homelands.”

The video, entitled 'A Message to America', is a stark message to US President Barack Obama warning of further retaliation to come – including the beheading of a second journalist.

The execution follows nearly two weeks of US air strikes that have pounded militant positions and halted the advance of Isis extremists, which until this month had captured a third of Iraq with little resistance.

The video has not been verified but the journalist's family have accepted that it shows his death and the Foreign Secretary said the footage appears to be genuine.

The clip begins with a recording of President Obama saying he had authorised strikes in Iraq.

"Obama authorises military operations against the Islamic State effectively placing America upon a slippery slope towards a new war front against Muslims," words appear in English and Arabic on the screen.

It shows black and white aerial footage of air strikes with text saying: "American aggression against the Islamic State"

A person identified as Mr Foley and wearing an orange outfit is seen kneeling in the desert as a man in black dress with a black mask stands beside him, holding a knife. "I call on my friends family and loved ones to rise up against my real killers, the US government, for what will happen to me is only a result of their complacency and criminality," the kneeling man says, in what appears to be a prepared statement.

The man in the mask says: "This is James Wright Foley, an American citizen of your country. As a government you have been at the forefront of the aggression towards the Islamic State,' begins the executioner.

"You have plotted against us and gone far out of your way to find reasons to interfere in our affairs. Today, your military airforce is attacking us daily in Iraq.

"Your strikes have caused casualties amongst Muslims. You are no longer fighting an insurgency.

"We are an Islamic army and a state that has been accepted by a large number of Muslims worldwide.

"So any attempt by you, Obama, to deny the Muslims their rights of living in safety under the Islamic Caliphate will result in the bloodshed of your people."

Following his statement he beheads the kneeling man.

The video's grisly message was unambiguous, warning of greater retaliation to come against Americans following nearly two weeks of US air strikes that have pounded militant positions and halted the advance of Islamic State, which until this month had captured a third of Iraq with little resistance.

The video goes on to show a man thought to be a second journalist, Steven Sotloff, who was kidnapped in Syria in August 2013, whose life the militant said depended on how the US responded.

After bringing out the second man and making him kneel, the masked executioner addresses the US President, saying: “The life of this American citizen, Obama, depends on your next decision.

The White House said last night that it had yet to confirm whether the man in the video was Mr Foley. Caitlin Hayden, a spokeswoman for the National Security Council, said in a statement, “The intelligence community is working as quickly as possible to determine [the video’s] authenticity. If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends.”

The mother of Mr Foley has urged his kidnappers to spare the lives of any remaining hostages.

In a message posted to the Facebook page “Free James Foley”, Diane Foley wrote: “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.”

Isis has declared an Islamic state – caliphate – across swathes of northern Iraq and Syria, taking numerous towns and cities. In recent days, Kurdish fighters, back by US air strikes have engaged in a battle for control of the key strategic Mosul dam.

Islamic State added new fighters in Syria at a record rate in July, according to the Syrian Observatory for Human Rights, which monitors the conflict. About 6,300 men - 80 percent of them Syrian and the rest foreigners - joined last month, Rami Abdelrahman, founder of the Observatory, told Reuters news agency.