Video posted online purports to show beheading of British aid worker by Islamic State group

WATCH: Islamic State has released a video they claim shows the execution of British hostage David Haines. Danielle Nottingham reports on the latest brutal killing by IS militants.

LONDON – British aid worker David Haines devoted his life to helping civilians in war zones and that is how he should be remembered, his family said Sunday as they grieved his death at the hands of Islamic State militants in Syria.

Haines – the third Westerner beheaded in recent weeks by the Islamic State group – had entered humanitarian work with enthusiasm, his brother Mike Haines said in a statement.

“He helped whoever needed help, regardless of race, creed or religion,” he said of his brother.

WATCH: Brother and friends react to killing of Briton by Islamic State.

Prime Minister Stephen Harper, in a tweet, condemned the killing.

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Haines was kidnapped in Syria in March last year when he was working for the French aid group Agency for Technical Cooperation and Development, or ACTED, to help victims of the fighting there.

He had also worked for groups such as Handicap International, which helps the disabled during conflicts, and Nonviolent Peaceforce, which sends unarmed peacekeepers into conflict zones. He had previously been in Libya during its civil war and South Sudan.

Mike Haines said he had also worked for the United Nations in the Balkans “helping people in real need.”

The leader of Bosnia’s Islamic Community, Husein Kavazovic, called on fellow Muslims Sunday to “show resolve to stop the murderers.” He said Haines’ family can be proud of his legacy.

READ MORE: Slain UK hostage’s family: Remember David Haines for trying to help victims in war zones

A video has been posted online purporting to show the beheading by the Islamic State group of British aid worker David Haines, who went missing in Syria last year.

The video emerged hours after the family of Haines issued a public plea on Saturday urging his captors to contact them.

WATCH: UK Prime Minister David Cameron denounced the killing of British aid worker David Haines as “an act of pure evil”

British Prime Minister David Cameron confirmed Haines’ death in a statement posted late Saturday on his official Twitter account after the British Foreign Office had said earlier that it was “working urgently to verify the video.”

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“The murder of David Haines is an act of pure evil,” Cameron said online. “My heart goes out to his family, who have shown extraordinary courage and fortitude.”

Cameron returned to his residence at Downing Street and is expected to chair a meeting of the government’s emergency response team on Sunday.

Islamic State militants previously beheaded American journalists James Foley and Steven Sotloff, as well as Kurdish and Lebanese soldiers, and posted video evidence online.

At the end of the last video showing the beheading of Sotloff, they threatened to kill Haines next and briefly showed him on camera.

The latest video’s desert setting is similar to that seen in the videos of Foley’s and Sotloff’s killings.  Like Foley and Sotloff, Haines kneels alongside a masked man holding a knife, and delivers a scripted message before he is killed.

“I would like to declare that I hold you,  David Cameron, entirely responsible for my execution,” Haines said, according to a transcript of the video posted online. “You entered voluntarily into a coalition with the United States against the Islamic State.”

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

The video’s final shot shows the executioner standing alongside and threatening to kill a man identified in media reports as British citizen Alan Henning.

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In a statement, United States President Barack Obama said the United States stands shoulder to shoulder with Britain in “grief and resolve.” The president said the U.S. would work with Britain and a broad collation of nations to bring to justice those who committed what he calls an “outrageous act.”

Repeating a message from his Wednesday night address laying out a strategy for attacking the Islamic State group, Obama says the coalition will degrade and destroy the threat the extremists pose to people around the world.

The 44-year-old Haines was abducted in Syria in 2013 while working for an international aid agency. He had worked for aid groups Nonviolent Peaceforce and the Agency for Technical Cooperation and Development, which describes its work as helping to rehabilitate populations riven by war, natural disasters, economic or social crises.

The British government had managed to keep Haines’ kidnapping secret out of concern for his safety until the most recent video identified him as a captive.

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