A screengrab taken from a video released by Isil shows British aid worker David Haines dressed in orange and on his knees in a desert landscape speaking to the camera before being beheaded by a masked militant. Image Credit: AFP

London: David Cameron has begun an emergency meeting in Whitehall after the Islamic State of Iraq and the Levant (Isil) released a video purporting to show the beheading of British aid worker David Haines.

The prime minister returned to Downing Street late on Saturday night, as Britain keeps its options open on whether to join the US in conducting air strikes against the Isil.

Ahead of the Cobra meeting, Cameron released a statement condemning the murder as an “act of pure evil”. He said the UK would do “everything in our power to hunt down these murderers and ensure they face justice, however long it takes”.

“This is a despicable and appalling murder of an innocent aid worker. My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal,” he added.

The US carried out at least nine air strikes last week, while the UK has pledged to help arm the Kurdish resistance forces in Iraq. These actions were both named in the video message, entitled A Message to the Allies of America, as reasons behind the killing of 44-year-old Haines.

The masked killer ended the video with a warning over a second British hostage, named in the international press and on social media as Alan Henning, a British aid worker.

Before his death, Haines read a statement holding Cameron directly responsible for his execution, while the Isil fighter addressed Cameron directly, saying Haines “has to pay the price for your promise”.

It is the third video released on the internet showing what appears to be the same Isil fighter with a UK accent beheading a hostage and threatening the next one. The Foreign Office said all the signs are that the video appears to be genuine. Two previous videos showed the murders of US journalists James Foley and Steven Sotloff.

The US president, Barack Obama, also condemned the “barbaric murder” of Haines. “Our hearts go out to the family of Mr Haines and to the people of the United Kingdom,” he said. “The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve. We will work with the UK and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world.”

The prime minister is due to make a last visit to Scotland this week but his attention may now be diverted away from the independence referendum towards events in the Middle East. Alex Salmond, the Scottish first minister, said the killing was “an act of unspeakable barbarism” and revealed he had called a separate Holyrood “resilience meeting”.

Cameron has promised to seek the support of parliament if the UK joins a military campaign against Isil, unless it is an emergency, last-minute action.

However, he is likely to need the support of Labour — which did not support last year’s government motion to back air strikes in Syria — as there is still unease about the idea of joining another war among some on the Conservative backbenches.

On Saturday night, Ed Miliband MP, the Labour leader, said acts such as the murder of Haines “will not weaken but strengthen the resolve of Britain and the international community to defeat Isil and their ideology”.

‘Warped logic’

“I am sickened at the disgusting, barbaric killing of David Haines,” he said. “He was somebody whose only purpose was to help innocent people, themselves victims of conflict. That Isil would choose to kill him says everything about their warped logic and murderous ways. My deepest condolences and thoughts are with his family as they cope with this terrible crime. And the hearts of the British people will go out to them.”

Douglas Alexander, the shadow foreign secretary, told Sky News’s Murnaghan programme that there was “no question that this is a threat that is going to have to be confronted — principally by those in the region”.

Sir Menzies Campbell, a former Liberal Democrat leader who strongly opposed the Iraq war, said it was not possible for the UK to rule out direct military action against Isil.

“You have to ensure, as I say, you’ve got a clear set of both political and military objectives,” he told LBC 97.3. “And there’s always the tendency — and remember, we’re not dealing with people who don’t understand these things. We’re dealing with highly sophisticated, sometimes highly educated, very clever people whose whole objective is to try to recreate what President Bush rather unwisely called ‘the war on terror’.

“They regard themselves as warriors, they regard themselves as being engaged in a war and of course as soon as you recognise that then you give them some justification and that’s what they’re out to try and provoke the British government into doing.”

Separately, Lord Dannatt, a former chief of the UK army, warned that the strength of Islamic State would grow if the government did not “confront and destroy” its influence.

British Muslims from the Islamic Society of Britain have objected to politicians calling the terrorist group Islamic State, saying it lends the group legitimacy and has become a slur on their faith.

On Sunday, Dr Shuja Shafi, the secretary-general of the Muslim Council of Britain, said British Muslims “condemn unreservedly the murder of our fellow Briton, David Haines”.

“Our deepest condolences, thoughts and prayers go out to the friends and family of Mr Haines,” he said. “David Haines went out to the region to help the people of the region. That extremists chose to murder him only shows once again the depravity of their warped ideology. They have killed so many innocent people in the region, and they perform such murders only to draw attention to their cause of destruction.

“These extremists in Iraq and Syria claim to be acting in the name of Islam. But there is nothing in our faith that condones such behaviour. Muslims in Britain and around the world have condemned these people, and the arguments they use have been refuted comprehensively as being far from the religion of Islam.”