The terrorist group ISIS tonight released a video purporting to show the recent beheading of a British humanitarian aid worker captured in Syria last year.

The video apparently shows the grisly murder of David C. Haines, 44, in an execution that British Prime Minister David Cameron called “an act of pure evil.”

Cameron was huddled with military and political aides in an emergency meeting at 10 Downing Street in London.

“The U.S. stands shoulder to shoulder with our close friend and ally.”[email protected]

In a statement released by the White House late tonight, President Obama called the act “barbaric,” saying:

The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve. We will work with the United Kingdom 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.

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Obama delivered a brief address Wednesday night on America’s intention to work with allies to “destroy” ISIS, the army of brutal Islamist jihadists in Iraq and Syria also known as the Islamic State and ISIL.

The terrorist group previously claimed responsibility for the videotaped beheadings of two American journalists in addition to the ongoing slaughter of hundreds of fellow Muslims as well as Christians and other religious minorities.

“This act of savagery underscores that we are at war with ISIS.”[email protected]

“This is an act of pure barbarism and savagery by Islamist militants that underscores the fact that we are at war with ISIS,” said Heritage Foundation foreign policy expert Nile Gardiner, a former aide to the late British Prime Minister Margaret Thatcher. “The terrorists responsible for this murder must be hunted down and destroyed.”

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Gardiner, who now directs Heritage’s Thatcher Center for Freedom, added:

Britain must join U.S.-led military action to defeat ISIS and clear its forces out of Iraq. This is a war against tremendous evil that must be waged and won by the free world. America needs allies in the fight against the Islamists, and has no greater friend than Britain.

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In the video, a man kneeling on the ground identifies himself as Haines and recites lines crafted to disparage Cameron and the UK. A hooded man next to him then says:

“This British man has to pay the price for your promise, Cameron, to arm the Peshmerga [Kurdish fighters in Iraq] against the Islamic State. If you, Cameron, persist in fighting the Islamic State then you like your master Obama will have the blood of your people on your hands.”

'Shoulder to shoulder': British Prime Minister David Cameron and President Obama. (Photo: Newscom)

‘Shoulder to shoulder’: British Prime Minister David Cameron and President Obama. (Photo: Newscom)

Steven Bucci, director of Heritage’s Allison Center for Foreign and National Security Policy, addressed the propaganda motives of this latest ISIS murder of a hostage, which follows the beheadings in recent weeks of two U.S. journalists, James Foley and Steven Sotloff.

“This is unlikely to drive a wedge between the U.S. and our strongest ally, the UK,” Bucci said. “It is obvious that ISIS does not remember other attempts to intimidate the British people — Hitler and the terror bombing of London, for example.”

“America must stand with the UK … and together destroy this murderous cult, ISIS.”[email protected]

Bucci,  a former Army Special Forces officer, added:

They don’t run from a fight, and will rise up to meet this horrible crime. America must stand with the UK in mourning their loss, and then together, destroy this murderous cult called ISIS.

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The Daily Telegraph, a leading London newspaper, reported that Haines, the father of two children, was employed by “aid agencies in some of the world’s worst trouble spots, including Libya and South Sudan.” The paper added:

He was travelling in a car through northern Syria in March last year when he was abducted along with an Italian colleague, who was later released. He was working for a French aid organization, Agency for Technical Cooperation and Development (Acted).

Mr. Haines is believed to have been kidnapped by a gang which later sold him on to ISIL [ISIS]. His identity remained a closely guarded secret for 19 months in a bid to increase the chances of a negotiated release.