President Barack Obama vowed that the US would stand "shoulder to shoulder" with the UK after the apparent murder of Briton David Haines. Islamic State militants released a video claiming to show his murder.
The US president issued a statement that strongly condemned "the barbaric murder" of Haines by the "Islamic State" (IS) terrorist group.
"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve," said Obama. "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."
British Prime Minister David Cameron had earlier described the apparent beheading as "an act of pure evil" and promised to hunt down the killers.
"This is a despicable and appalling murder of an innocent aid worker," said Cameron in a statement released by Downing Street. He vowed that the "Islamic State" (IS) members responsible would be caught "however long it takes" - a sentiment he also shared via Twitter.
Britain's foreign ministry had earlier said it was working to confirm the authenticity of the video - believed to have been filmed in Syria - that claimed to show the killing.
IS on Saturday claimed the beheading in what would be the third such execution in recent weeks.
ISIS anger over airstrikes
The video, "A Message to the Allies of America," blames British Prime Minister David Cameron for entering a coalition with the United States, which has said it is at "war" with the IS and has launched air strikes against them in Iraq.
"You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor Tony Blair did, following a trend amongst our British prime ministers who can't find the courage to say no to the Americans," Haines said in a prepared statement in the video.
Haines, a father of two from Perth in Scotland, was kidnapped in Syria in March 2013 while working for the Agency for Technical Cooperation and Development (ACTED), a French relief charity. He had previously been involved in humanitarian work in other parts of the Middle East as well as the Balkans and Africa.
Between 1999 and 2004 , Haines worked for Arbeiter Samariter Bund (ASB), a Germany charity carrying out reconstruction work in post-war Croatia.
Steven Sotloff and fellow US journalist James Foley were also kidnapped in Syria. IS released a video claiming it had executed Foley on August 19, with the video of Sotloff's death coming two weeks later on September 2.
rc/crh (AP, AFP, Reuters)