Prime Minister David Cameron today cut short his holiday in Cornwall for meetings on Iraq and Syria after the apparent beheading of a US journalist by an Islamic State militant who spoke with a British accent.
Cameron described the killing as "shocking and depraved".
He cut short his holiday in Cornwall for meetings on Iraq and Syria with UK foreign secretary Philip Hammond, Downing Street said.
Hammond said a video of the killing of 40-year-old James Foley appeared to be genuine.
He had been missing since he was seized in Syria in 2012.
Hammond said that the UK government was aware of "significant numbers" of British nationals involved with extremist organisations overseas.
He acknowledged the militant spoke with a British accent but said further analysis of the footage was needed.
"We're absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with Isil [now known as IS] and other extremist organisations," he said.
"This is something we have been tracking and dealing with for many many months and I don't think this video changes anything.
It just heightens awareness of a situation which is very grave and which we've been working on for many months," he added.
It is estimated that IS, which has seized large parts of northern Iraq and Syria, has up to 400 recruits from the UK in its ranks.
Some 69 people suspected of Syria-related jihadist activities have now been arrested in the UK.
