British Prime Minister David Cameron said Sunday he will work with allies to defeat the terrorist group Islamic State, and that countries must “drain this poison from our society,” after the militants released a video showing the beheading of British aid worker David Haines.

Cameron, who was attending a wedding on Saturday, raced back to London overnight to chair a meeting of his security Cabinet, including his advisers, officials and intelligence chiefs. The Prime Minister described the murder of Haines as "an act of pure evil" in a tweet from his official account. 

The 44-year-old Haines was abducted in Syria in 2013 while working for an international aid agency. The British government had managed to keep his kidnapping secret out of concern for his safety until another video recently released by Islamic State, also known as ISIS, identified him as a captive.

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“We must confront this menace. We must drive back, dismantle and ultimately destroy [ISIS],” Cameron said in his remarks Sunday.  He reiterated that Islam is a religion of peace, and said of the militants, “they are not Muslims; they are monsters.”

He warned against being complacent, advising that “the problem will merely get worse, not just for us, but for Europe and the world.” He said that Britain would not work to destroy Islamic State on its own, but will be “working with others to extinguish this terrorist threat.” Cameron said  Britain supports the U.S. decision to take direct military action.

The British Foreign Office said late Saturday that it was "working urgently to verify the video," but Cameron's statement left no room for ambiguity. Fox News confirmed that the tape is being examined by the National Counterterrorism Center in Washington.

"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes," Cameron added.

In a statement released late Saturday, President Obama condemned the murder as "barbaric."

"The United States strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group ISIL. 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," the statement read.

"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."

In the video, the victim identifies himself as Haines before addressing the camera.

"I would like to declare that I hold you David Cameron entirely responsible for my execution," the victim says. 

"You entered voluntarily into a coalition with the United States against the Islamic State just as your predecessor Tony Blair did, following a trend against our British prime ministers who can't find the courage to say 'no' to the Americans."

After he speaks, the executioner dressed in black, speaking with an English accent, addresses Cameron.

"This British man has to the pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State," the militant says. "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."

The video emerged a day after Haines' family issued a public plea late Friday urging his captors to contact them. The Foreign Office said it was offering Haines' family "every support possible. They ask to be left alone at this time."

Islamic State militants have beheaded two American journalists, James Foley and Steven Sotloff, as well as Kurdish and Lebanese fighters, and posted video evidence online. At the end of the last video showing the beheading of  American journalist  Steven Sotloff, the Islamic State group threatened to kill Haines next and briefly showed him on camera.

In the video posted Sunday, the group threatened to kill another Briton. Both British men were dressed in orange jumpsuits against an arid Syrian landscape, similar to that seen in the Foley and Sotloff videos.

The SITE Intelligence Group, a U.S. terrorism watchdog, reported the video, which was also posted online by users associated with the Islamic State group.

Mike Haines said in a statement issued by the Foreign Office that his brother was devoted to humanitarian work.

"David was most alive and enthusiastic in his humanitarian roles," Mike Haines said. "His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair. He was and is loved by all his family and will be missed terribly."

At the time of his abduction, Haines was working for the French international aid agency ACTED which was helping thousands of Syrians displaced by the fighting between Syrian President Bashar Assad's forces and rebel groups, including the ISIS, seeking to oust him. ACTED said in a statement earlier this month that Haines had worked with as an international aid worker since 1999, helping victims of conflict in the Balkans, Africa and the Middle East. Before heading to Syria, ACTED said Haines had worked in 2012 as a "non-partisan unarmed civilian peace facilitator" in South Sudan.

British media have reported that Haines, who is married to a Croatian woman, was educated in Scotland, and spent 11 years serving in the British Royal Air Force before becoming an international aid worker.  Britain's Press Association said Haines had a teenage daughter in Scotland from a previous marriage and a 4-year-old daughter from his present marriage.

Fox News' Catherine Herridge and The Associated Press contributed to this report.