REUTERS could not immediately verify the footage. However, the images were consistent with that of the filmed executions of two American journalists, James Foley and Steven Sotloff, in the past month.
Haines, a 44-year-old father of two from Perth in Scotland, was kidnapped last year while working for the French agency ACTED.
The video entitled, ‘A Message to the Allies of America,’ opened with UK Prime Minister David Cameron talking about working with the Iraqi government and allied Kurdish Peshmerga forces to defeat Islamic State.
“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 was shown and the masked man said he would be killed if Cameron continues to support the fight against Islamic State.
Cameron condemned the killing and said 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.
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.
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 US President Barack Obama which have been dismissed as scripted by Islamic State 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.
The United States resumed air strikes in Iraq in August for the first time since the withdrawal of the final US troops from the country in 2011.
The raids followed major gains by Islamic State, 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 Islamic State and has said the US 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 US air strikes and Cameron has repeatedly said Britain has ruled nothing out except combat troops on the ground.
Haines’ family appealed earlier on Saturday to his captors to respond to their messages.
“We are the family of David Haines,” relatives said in a statement released by Britain’s Foreign Office. “We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us.”
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. â€“ Reuters