American and British intelligence agencies are working to identify the man "with a British accent" who appeared to kill American journalist James Foley in a video posted by ISIS.
"Agencies on both sides of the Atlantic, are first of all looking to authentic the video ... and then to see if we can identify the individual in question," British Foreign Secretary Philip Hammond told ITV News.
"If [the reports are] true, the murder of James Foley is shocking and depraved," British Prime Minister David Cameron said.
Islamic State militants who released a video purportedly showing the beheading of American journalist James Wright Foley, have threatened to kill another man, identified as American journalist Steven Joel Sotloff.
A man identified in the video as Joel Sotloff is shown at the end of the video after Foley’s apparent beheading.
Foreign editor for ABC News America, Jon Williams, tweeted that Sotloff’s family had been given advice to avoid publicising the news of Sotloff’s kidnapping.
Sotloff’s last tweet was posted on August 3, 2013, the day before he is believed to have been kidnapped. In the tweet, he mentioned being attacked by riot police with pepper spray in the southern Turkey town of Antakya on July 31.
In the five-minute video, distributed online by known Islamic State sources, the group declares that James Foley was killed after US President Barack Obama ordered air strikes against IS positions in northern Iraq.
James Foley had been missing since he was seized by armed men in Syria in November 2012.
The group has threatened to kill another reporter, believed to be US man Steven Sotloff who is shown at the end of the video, unless US air strikes are stopped.
'It's as near to pure evil that we will ever possibly see'
The purported execution is carried out in an open desert area with no immediate signs as to whether it is in Iraq or Syria, by a black-clad masked militant who speaks in English with a British accent.
Hunt to identify ISIS executioner
Britain’s foreign secretary Philip Hammond said intelligence agencies are examining a video, posted by ISIS militants showing a beheading, for clues of the identity of the executioner.
"We have been saying for a long time that there are a significant number of British nationals in Syria and Iraq operating with extremist organisations," he told the BBC.
"That’s one of the reasons why this organisation represents such a direct threat to the UK’s national security.
"Many of these people may seek at some point to return to the UK and they would then pose a direct threat to our domestic security.
"...This is a poison, a cancer, what’s going on in Iraq and Syria and it risks spreading to other parts of the international community and affecting us all directly."
Meanwhile, French President Francois Hollande said he would next month arrange an international conference on the threat posed by Islamic extremists in Iraq, describing the current international situation as "the most serious since 2001".
"I think we are in the most serious international situation since 2001 ... I will therefore propose an initiative on security in Iraq and the fight against Islamic State, from September," Hollande told Le Monde daily.
The FBI previously reported that Foley was taken by an "organized gang" after leaving an internet café in Binesh, Syria.
Foley's translator, employed to help him travel across the Syrian-Turkish border, was also taken but later released.
US intelligence has been attempting to verify the video, the White House said, adding that it would be "appalling" if genuine.
His family have taken to Twitter, urging people not to watch the video still being circulated.
"Please honor James Foley and respect my family's privacy. Don't watch the video. Don't share it. That's not how life should be," Foley's sister, Kelly, wrote.
Meanwhile, Twitter chief executive Dick Costolo says the network is trying to block images and video of Foley’s killing.
"We have been and are actively suspending accounts as we discover them related to this graphic imagery," he wrote.
Tributes pour in for James Foley
The mother of American freelance journalist James Foley says he was an "extraordinary person" who "gave his life trying to expose the world to the suffering of the Syrian people".
Diana Foley also issued an appeal to the Islamic State kidnappers who claimed responsibility for the act.
"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," she said in a message posted on Facebook page "Free James Foley".
Condolences have also been posted online.
Foley had reported from the Middle East, covering issues across Iraq, Libya and Syria.
He had previously been held captive for several weeks in Libya after being kidnapped while covering the civil war in 2011.
Foley's parents previously spoke out about his disappearance to the GlobalPost, a former employer of the freelance journalist.
His father John told the new outlet in 2013 that they didn't know who had detained their son.
"But we beg his captors to recognize that our son is an innocent, objective journalist who has done no harm to anyone and to please set him free," he said.
"Find James Foley," the campaign run by his family to secure the 40-year-old freelancer's release, posted a public message online following the release of the video asking for time "to seek answers."
Video is sickening, pure evil: Abbott
Australian Prime Minister Tony Abbott has labelled the reports as sickening.
"It's absolutely sickening, it's absolutely despicable and it's a sign that there can be no compromise," he said.
"It's as near to pure evil that we will ever possibly see."
Treasurer Joe Hockey later urged Australians to remain united.
"Terrorism is not defined by faith," he said.
"Terrorism is defined by the actions of an individual."