British hostage: Reaction to David Haines killing
The killing of British hostage David Haines by Islamic State militants has been condemned by a range of international leaders and faith figures.
UK prime minister David Cameron described it as an "act of pure evil" and pledged to hunt down those responsible.
US President Barack Obama said: "Our hearts go out to the family of Mr Haines and to the people of the United Kingdom.
"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve.
"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."
Mr Haines' employers, the Acted aid agency, said the murder must not go unpunished.
"David was appreciated by the Acted team and all those around him, notably for his generosity, commitment, and his professionalism," the statement said.
"The horrible assassination of David, an aid worker, goes against all humanitarian principles and is a crime against humanity. This barbaric crime must not remain unpunished."
Australian Prime Minister Tony Abbott said Mr Haines's murder was "further demonstration that this particular terrorist group does not just do evil, but exults in doing evil".
The office of the French president, Francois Hollande, said: "The heinous murder of David Haines shows once again how the international community must mobilise against [IS]."
Scottish First Minister Alex Salmond said the release of the video "demonstrated a degree of brutality which defies description".
"It should be remembered that Mr Haines was in the region as an aid worker helping local people," he added.
The leader of the Better Together campaign in Scotland, Alistair Darling, said: "David Haines was trying to help people in dreadful conditions".
His death was "inexcusable, a barbaric act" that would "strengthen the resolve of the international community," he added.
Labour leader Ed Miliband said he was "sickened at the disgusting, barbaric killing" of Mr Haines.
The Deputy Prime Minister, Nick Clegg, said the government "will not rest" until the killers of David Haines "face justice".
Pete Wishart, who is the MP for Perth, where Mr Haines went to school, said of his family: "It must be unimaginable for them, so my heart really goes out to them this morning."
The Archbishop of Canterbury, Justin Welby, posted on Twitter: "In every church let us pray for the family of David Haines, evilly killed in the place he was serving in love for its suffering people."
The killing was also condemned by Muslim leaders in Britain.
Secretary General of the Muslim Council of Britain Dr Shuja Shafi said: "David Haines went out to the region to help the people of the region.
"That extremists chose to murder him only shows once again the depravity of their warped ideology."
Dr Qari Asim, Imam of Leeds Makkah Mosque, a Sunni scholar, said: "This vile and unwarranted terrorist attack is the action of cowards."
The president of the Islamic Society of Britain, Sughra Ahmed, said: "If someone who commits such evil and such heinous crimes calls themselves the Islamic State, then we need to understand actually that there's nothing Islamic and there's nothing state-like or legal about the work that they're doing."
Sayed Ali Abbas Razawi, from Majlis-e-Ulama, which represents Shia Muslims in the UK and Europe, said militants were hiding behind a "false interpretation" of Islam, describing the group as "criminals and villains".