British Secretary of State for Foreign and Commonwealth Affairs Philip Hammond has described the apparent beheading of a US journalist by an Islamic State militant with a British accent as "an appalling example of the brutality of this organisation".
The foreign secretary said a video of the killing appeared to be genuine.
He added that the government was aware of "significant numbers" of British nationals involved with extremist organisations overseas.
James Foley, 40, had been missing since he was seized in Syria in 2012.
Hammond told BBC Breakfast that while the video, posted online, had not been verified, "All the hallmarks point to it being genuine".
He acknowledged the militant spoke with a British accent but said further analysis of the footage was needed.
"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 added.
"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."