British prime minister David Cameron broke off his holiday on Wednesday for talks on the threat posed by Islamic State jihadists following the "shocking and depraved" apparent beheading of US journalist James Foley.
Cameron was returning to his Downing Street office in London to discuss the situation in Iraq and Syria as experts worked on trying to identify the executioner in the beheading video, who spoke with a British accent.
"If true, the murder of James Foley is shocking and depraved. I will today chair meetings on the situation in Iraq/Syria," Cameron said on Twitter.
The video posted Tuesday on the Internet showed a masked militant beheading a man resembling Foley, who has been missing since he was seized in Syria in November 2012.
Downing Street said in a statement that Cameron would meet with foreign secretary Philip Hammond and other senior officials "to discuss the situation in Iraq and Syria and the threat posed by ISIL (Islamic State) terrorists".
Read: ISIS releases 'video showing beheading' of US journalist
Wednesday was to have been the second day of the prime minister's break in Cornwall, southwest England.
Hammond voiced his "absolute horror" at the apparent beheading, which he said is "just is one more example in a catalogue of brutality by this organisation," referring to IS.
He said "atrocities" carried out by IS as it has seized swathes of territory in Iraq and Syria were driving the international community's efforts to support Iraqis and Kurds in pushing back against the "evil organisation".
Brits 'most vicious fighters'
He also said the apparent attack showed the security threat posed by European extremists fighting with IS if they attempted to return home.
The masked executioner in the video speaks English with a British accent.
"On the face of it, it appears to have been a British person. We'll have to do some more analysis to make quite certain that that is the case," Hammond said.
Around 400 Britons are believed to have gone to Syria over the last two years.
Cameron on Sunday said Britain would send military support to forces fighting IS, warning that not acting could allow the "terrorist movement" to "target us on the streets of Britain".
Hammond said there were already "significant" numbers of British nationals in Syria and an increasing number in Iraq, which could pose a "direct threat to our own national security" if they seek to return with the "tradecraft" they have learnt in those conflicts.
British extremists are among the "most vicious and vociferous fighters" in the IS ranks, said Shiraz Maher, an expert on jihadism.
Maher, a senior research fellow at the International Centre for the Study of Radicalisation at King's College London, said they had been operating as suicide bombers and executioners.
"Foreign fighters going out to Syria are not going out there to be backseat riders. They are going out to be full participants in the war, to be at the forefront of the conflict," he told AFP.
He said he and his colleagues were "fairly satisfied that it's a Brit" in the video "due to the colloquialisms" used, as well the accent.
"We have a database of several hundred fighters in Syria and myself and my colleagues are frantically looking through it to try to pin him down," he added.
Read: Mother mourns US journalist purportedly beheaded by ISIS militants