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, has been missing since he was seized in Syria in 2012.
Hammond told BBC Breakfast that while the video, which was 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’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.”
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.
Hammond said the involvement of British nationals with extremist groups was one of the reasons that IS “represents such a direct threat” to the UK’s security.
He added: “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.”
That would be especially likely if IS was “pushed back”, leaving its foreign fighters to return to their country of origin, said Hammond.
“We would see these people going back and potentially carrying on their fighting in our own homelands,” he added.
In the video, titled A Message to America, a man who appears to be Foley is wearing an orange jumpsuit, kneeling in desert-like terrain beside an armed man dressed in black.
The masked militant, who speaks with a British accent and identifies himself as an IS member, says the journalist’s death is retaliation for US airstrikes against IS targets in Iraq.
The foreign secretary felt “absolute horror” at the “brutal execution”, he told BBC Radio 4′s Today programme.
The White House said if the video was genuine, the US would be “appalled by the murder of an innocent American journalist”.
Foley’s mother Diane said on Facebook she was proud of her son, who had reported extensively across the Middle East, working for America’s Global Post and other media outlets including the French news agency AFP.
“He gave his life trying to expose the world to the suffering of the Syrian people,” she wrote.
Shiraz Maher, from King’s College London’s International Centre for the Study of Radicalisation, said that British IS recruits were “not there to take a back seat role” and that many were becoming “hardened killers”.
He told Today: “Unfortunately the British participation in the conflicts now raging in both Syria and Iraq has been one of full participation, one that has seen them at the front lines, taking part in the conflict in every way.
“So we have seen British fighters out there operating as suicide bombers; we have seen them operating as executioners.
“Unfortunately they are amongst some of the most vicious and vociferous fighters who are out there.” BBC
