Canadian Prime Minister Stephen Harper and U.S. President Barack Obama joined British Prime Minister David Cameron on Saturday in condemning the apparent killing of Briton David Haines by ISIS militants after the group released a video purporting to show the beheading of the aid worker.

"Canada Condemns the barbaric killing of David Haines by ISIL.  Our thoughts & prayers are with his family at this time," Harper tweeted.

"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve," Obama said in a statement.

"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," he added.

Obama is calling for a coalition of Western and Middle Eastern countries to fight ISIS. The group in the past month released videos showing the beheadings of American journalists James Foley and Steven Sotloff.

Another hostage, identified as Alan Henning, was shown at the end of the video. The apparent executioner said he would be killed if the U.K. continues to support the fight against ISIS.

Haines, a 44-year-old father of two from Perth in Scotland, was kidnapped last year while working for the French agency ACTED. 

Cameron was the first to react, condemning the killing and saying he would bring the killers to justice.

"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil. My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal," he said in a statement released by Downing Street.

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

'A Message to the Allies of America'

Steven Sotloff

A threat against Haines was made at the end of a video released earlier this month showing the beheading of U.S. journalist Steven Sotloff, above. (Associated Press)

The video entitled, "A Message to the Allies of America," opened with Cameron talking about working with the Iraqi government and allied Kurdish Peshmerga forces to defeat ISIS.

"This British man has to pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State," said a masked man dressed in black with a British accent, standing over Haines, who was shown kneeling and wearing an orange jumpsuit.

The video then showed the beheading of the kneeling man.

At the end of the video, another hostage, identified as Alan Henning, was shown and the masked man said he would be killed if Cameron continues to support the fight against ISIS.

In Washington, White House National Security Council spokeswoman Caitlin Hayden said she had no comment on the video and referred queries to the British government.

Forced confessions on video

In the video, Haines also spoke, saying Cameron was responsible for his execution. "You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor, Tony Blair, did," he said.

"Following a trend amongst our British Prime Ministers who can't find the courage to say no to the Americans. Unfortunately, it is we, the British public, that will in the end pay the price for our Parliament's selfish decisions."

Foley and Sotloff made similar speeches to U.S. President Barack Obama, which have been dismissed as scripted by ISIS and delivered under duress.

The purported executioner appeared to be the same British-accented man who appeared in videos with Foley and Sotloff, and it showed a similar desert setting. In both videos, the captives wore orange jumpsuits.

Air strikes resume

Journalist Slain James Foley

Like Haines, Foley, above, was forced to recite a confession before he was beheaded. (Nicole Tung/Associated Press)

The United States resumed air strikes in Iraq in August for the first time since the withdrawal of the final U.S. troops from the country in 2011.

The raids followed major gains by ISIS, which has declared an Islamic caliphate in areas it controls in Syria and Iraq.

Obama is now calling for a coalition of Western and Middle Eastern countries to fight ISIS and has said the U.S. intends to bomb Islamic State positions in Syria.

Britain has delivered humanitarian aid, carried out surveillance, given weapons to Kurds and promised training in Iraq. On military action, Britain supports U.S. air strikes and Cameron has repeatedly said Britain itself has ruled nothing out except combat troops on the ground.

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."
 

Paris-based ACTED has previously said Haines had been engaged in humanitarian work since 1999, helping victims of conflicts in the Balkans, Africa and the Middle East and that he was taken hostage in March 2013 in Syria. 

British media have reported that Haines, who is married to a Croatian woman, was educated in Perth, 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 fouryear-old daughter from his present marriage.