Foreign Secretary Philip Hammond has described a video showing the apparent beheading of a US journalist by an Islamic State fighter as "an appalling example of the brutality of this organisation".
Mr Hammond said the video's "hallmarks point to it being genuine", and acknowledged that the fighter who killed James Foley appeared to have a British accent.
"Certainly at first pass that's what it looks like and we will obviously want to investigate that further," Mr Hammond told the BBC.
"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.
"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."
A British family have told ITV News of their devastation after discovering a close relative is fighting with the Islamic State in Iraq.
A French journalist who was held hostage with James Foley in Syria has told ITV News it was "devastating" to see his murder.