Photo: ReutersIslamic State jihadists have released a video in which a militant speaking English with a British accent beheads a man they claim to be an American journalist.
The journalist, claimed by Islamic State to be James Foley, has been missing since he was seized by armed men in Syria in 2012.
The terrorist group, formerly known as Isis, released the graphic video of the execution, saying it was conducted in retaliation for US air strikes in northern Iraq.
The executioner then issued a threat to President Barack Obama that a second journalist would be killed unless air strikes are called off.
White House National Security Council spokeswoman Caitlin Hayden said: "We have seen a video that purports to be the murder of US citizen James Foley... the intelligence community is working as quickly as possible to determine its authenticity.
"If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends."
The video, which could not be independently verified, was posted on YouTube on Tuesday entitled "A Message to #America (from the #IslamicState)". It shows a shaven-headed man in an orange robe on his knees.
He was made to recite a statement calling the US government “my real killers” before he was beheaded.
Foley was working as a freelance journalist covering the Syrian civil war when he disappeared 21 months ago.
The video then showed a second prisoner, who the group named as Steven Sotloff, a US journalist missing since last August. The executioner said that “the life of this American citizen, Obama, depends on your next decision”.
The killer’s apparent distinct British accent will raise fears fears about the role of Islamic extremists from the UK in Islamic State, which now controls swathes of Syria and Iraq.
An official at the National Security Council said that US intelligence officials were trying to confirm the authenticity of the video.
Mr Foley, whose parents live in New Hampshire, turned 40 in captivity last year. His family previously believed that he was held by pro-government Syrian forces near Damascus.