Islamic State jihadists have released a video which they claim shows the beheading of a 44-year-old British aid worker, David Haines, who has been held hostage since his abduction last year.

The masked man in the latest video released across social media on Saturday night claims that Haines was killed because of UK Prime Minister David Cameron’s pledge to arm Kurdish forces in order to battle the jihadist group, according to reports.

“This British man has to pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State,” the executioner said, according to liveleak.

“My name is David Cawthorne Haines,” the hostage reportedly said before the execution.

“I would like to declare that I hold you, David Cameron, entirely responsible for my execution. You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor, Tony Blair, did, following a trend amongst our British Prime Ministers who can't find the courage to say no to the Americans.”

The video footage was similar to that of the executions of two American journalists, James Foley and Steven Sotloff, by Islamic State militants.

The murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude. — David Cameron (@David_Cameron) September 13, 2014

The UK Foreign Office confirmed Sunday that the video was genuine.

"All the signs are that the video is genuine. We have no reason to believe that it's not," a spokesman told AFP.

We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes. — David Cameron (@David_Cameron) September 13, 2014

The British PM vowed to “hunt down” the militants responsible for the murder of a "British hero" following an emergency meeting of the Cabinet Office Briefing Room (COBRA) on Sunday.

Cameron pledged to “bring them to justice, no matter how long it takes.” The IS militants are “not Muslims, they are monsters,” he said, adding that the UK cannot afford to ignore the threat that the extremists pose to international and domestic security.

“But ultimately, our security as a nation, the way we go about our everyday lives in this free and tolerant society that is Britain, has always depended on our readiness to act against those who stand for hatred and now stand for destruction. And that is exactly what we will do.”

Hours before the news of the execution emerged, Haines’ family had asked the hostage takers to contact them to discuss ransom for the British health worker, which IS had previously demanded.

“We are the family of David Haines. We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us,” a statement on behalf of the family by the Foreign and Commonwealth Office read.

I am sickened at the disgusting, barbaric killing of David Haines. — Ed Miliband (@Ed_Miliband) September 13, 2014

The 44-year-old Brit was seized by the Islamic State militants in Syria in March 2013 while employed by the Agency for Technological Cooperation and Development (ACTED). The father of two was taken hostage alongside his Italian colleague, the organization's coordinator, Federico Motka, on their way to the Turkish border.

READ MORE:US threatened Foley family over Islamic State ransom

Motka, who allegedly shared a cell with Haines, was freed in May after Rome authorized a ransom payment to the Islamic State.

Haines has made a successful career as a humanitarian aid worker since 1999 and has worked in conflict zones in Africa, the Balkans and the Middle East. Haines’ employer said last week in a statement: “A man's life should never be threatened on account of his humanitarian commitment.”

At the end of the nearly two-and-a-half minute long video clip that allegedly shows the beheading of Haines, another hostage was paraded who could be next on the Islamic State kill list. The captive, standing on his knees and dressed in the sadly all-too-familiar orange jumpsuit, was identifiable as Alan Henning, according to Reuters.

Scottish First Minister Alex Salmond said that the release of the video had “demonstrated a degree of brutality which defies description”.

In the meantime, US President Barack Obama has also condemned the “barbaric murder of UK citizen.”

“Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom. The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve,” he said in statement released by the White House.

READ MORE:US ‘at war’ with ISIS: White House and Pentagon upgrade ‘Al-Qaeda style’ op