LONDON - British Foreign Secretary Philip Hammond on Wednesday voiced his "absolute horror" at the apparent beheading of US journalist James Foley, which he said showed the "brutality" of Islamic State jihadists.
Hammond said the executioner in the beheading video appeared to be British and work was under way to verify his background.
He also said European extremists fighting with IS posed a security threat if they attempted to return home.
Hammond told BBC radio his reaction was "Horror, absolute horror at what appears to be a brutal execution.
"It just is one more example in a catalogue of brutality by this organisation." He said IS "atrocities" in Iraq and Syria were "completely horrifying and shocking" and were driving the international community's efforts to support Iraqis and Kurds in pushing back against the "evil organisation".
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.
"There are significant numbers of British nationals in Syria, increasingly in Iraq," he said, who pose a "direct threat to our own national security" if they seek to return with "the tradecraft that they've learned working with these terrorist organisations".
Around 400 Britons are believed to have gone to Syria over the last two years.
British extremists are among the most "most vicious and vociferous fighters" in the IS ranks, a jihadism expert has said.
Shiraz Maher, a senior research fellow at the International Centre for the Study of Radicalisation (ICSR) 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 said.
Speaking about the execution video, he added: "We're fairly satisfied that it's a Brit. We interact a lot with fighters in the region, to us it's pretty clear that this guy is most likely British, due to the colloquialisms as well (as 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."