The Islamic State of Iraq and Syria (ISIS) jihadist group claimed Saturday it executed British aid worker David Haines, in retaliation for British leader David Cameron entering a coalition with the United States against the militants.
This would be the third such execution in recent weeks, after two US journalists taken hostage in Syria were shown murdered.
The Islamist group released a video, available on the website of private terrorism monitoring group SITE, purportedly showing a masked militant beheading Haines.
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 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.
Responding late Saturday night, Cameron called the murder an "act of pure evil" and vowed Britain would do all in its power to bring the killers to justice.
"This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil," he said in a statement released by his Downing Street office.
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
President Barack Obama strongly condemned "the barbaric murder."
"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."
On Friday, Haines' family appealed to his captors to respond to their messages.
A 44-year-old father of two from Perth in Scotland, Haines was kidnapped last year while working for the French agency ACTED.
Earlier this month, he was shown kneeling in the sand wearing an orange jumpsuit in an IS video which showed the beheading of U.S. hostage Steven Sotloff.
The video contained a warning from a militant speaking with a British accent that Haines would be the next to be killed if Washington continued to launch air strikes against IS fighters in northern Iraq.
"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.
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 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 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.