UK Prime Minister David Cameron has interrupted his holiday to return to London after a video released by the Islamic State (IS) group purported to show a man with a British accent executing US journalist, James Foley.
Cameron will meet with officials from the Home Office, Foreign Office, and intelligence agencies on Wednesday to discuss the situation in Iraq and Syria, and the threat posed by fighters from the IS.
The footage posted late on Tuesday, showed Foley wearing orange overalls and kneeling besides an Islamic State group fighter, who warned Western countries against further intervention in Iraq.
The unidentified man spoke in what sounded like an English accent, before apparently beheading Foley in the video, titled "A Message To America."
"If true, the brutal murder of James Foley is shocking and depraved," said a statement from Cameron's office, the Reuters news agency reported.
"Our intelligence services will be looking very carefully on both sides of the Atlantic at this video to establish its authenticity, to try to identify the individual concerned and then we will work together to try to locate him," UK Foreign Secretary Philip Hammond told British broadcaster, Sky news, in a reference to the masked man.
"We are absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities."
Elsewhere, the UN Secretary General Ban Ki-Moon, said he condemned Foley's execution in the "strongest terms."
Foley's mother, Diane Foley, released a statement through a Twitter account the family had used to campaign for his release in which she confirmed his death and said he died "trying to expose the world to the suffering" of the Syrian people.
We implore the kidnappers to spare the lives of the remaining hostages. Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world.
"We implore the kidnappers to spare the lives of the remaining hostages. Like Jim, they are innocents," the statement read.
"They have no control over American government policy in Iraq, Syria or anywhere in the world."
The group claimed in the video to be holding another US journalist and said his life depended on US President Barack Obama's next move.
"The life of this American citizen, Obama, depends on your next decision," said the masked man, as he held a prisoner the video named as Steven Sotloff.
Foley, 41, was contributing videos to the AFP news agency for the media company GlobalPost before he was kidnapped on November 22, 2012 by unidentified gunmen.
"We have seen a video that purports to be the murder of US citizen James Foley by ISIL," Caitlin Hayden, a spokesperson for the US National Security Council, said on Wednesday.
"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.
"We will provide more information when it is available," she said.