JOHANNESBURG -The beheading of a third hostage by the Islamic State in Syria has been met with anger around the world.

A video has been released showing the beheading of David Haines, a British aid worker who was abducted in Syria last year.

His death follows the killing of two American journalists James Foley and Steven Sotloff taken hostage in Syria.

Haines is the third Westerner beheaded in recent weeks by the Islamic State group, which has seized vast areas in Syria and Iraq.

The video emerged a day after Haines' family issued a public plea late on Friday, urging his captors to contact them.

Britain and the United States have since vowed to destroy the movement in Iraq and Syria.

And Australian Prime Minister Tony Abbott has just announced he will deploy 600 troops to the United Arab Emirates to join the U.S.-led international coalition gearing up for war against the Islamic State.

CNN's Nic Robertson says, "While the British Foreign Office says that they're working urgently to verify this video, they've been able to reach out to Haines's family that they were not aware at that time that they'd done, so this is very much the early stages."

Earlier today British Prime Minister David Cameron condemned his slaying as 'an act of pure evil'.

At the same time United States President Barack Obama has expressed solidarity with ally Britain, after Haines' beheading.

US Secretary of State John Kerry is currently traveling to meet with his country's allies, hoping to forge a broad international coalition against ISIS in Iraq and Syria, which will include Arab nations.

Meanwhile, the executioner in the video slammed Britain for its stance on Syria and Iraq.

"You're even allies of America, which continues to strike the Muslims of Iraq and most recently formed [a coalition] will only accelerate more destruction.

In the video the executioner makes specific reference to the Prime Minister, saying his decisions have fuelled a war.

"Playing the role of the obedient lap dog, Cameron will only drag you and your people into another bloody and unwinnable war."

Additional reporting by CNN.

SOCIAL MEDIA REACTION

RIP David Haines - a lad who dedicated his life to aid work
May his cowardly murderers rot in hell

— Brian Durand (@BrianDurand56) September 14, 2014

A fitting memorial to David Haines would be a lengthy silence from politicians & pundits who routinely & ignorantly condemn foreign aid.

— James O'Brien (@mrjamesob) September 14, 2014

My thoughts & prayers are with the family & loved ones of David Haines, a brave man who worked in the service of the vulnerable

— Kristalina Georgieva (@KGeorgievaEU) September 14, 2014

RIP David Haines pic.twitter.com/QeS4pAot2y

— Historical Pics (@HistoricalPics) September 14, 2014

Only the greatest coward murders a man whose hands are tied behind his back. Let's remember David Haines for how he lived, now how he died.

— Jake Humphrey (@mrjakehumphrey) September 14, 2014

What a terrible piece of news to hear that Brit David Haines has been beheaded by that terror group
There are some horrible ppl in the world

— Anthony Ogogo (@AnthonyOgogo) September 14, 2014