Perhaps most ominously from a British point of view, in the video which claimed to show the beheading of Steven Sotloff, there is also now an additional threat. That it [Islamic State] will kill - behead - a British hostage, who it allegedly holds in its hands. There is real concern now for the British security services about whether the Islamic State might carry out this latest threat.
I've been told at the Nato summit that the intelligence services know exactly who 'Jihadi John' is - or at least are 99.9% certain.
The British aid worker being held by Islamic State is David Haines, a 44-year-old father-of-two from Perth, Scotland.
In a video statement which claims to record his final words US hostage and journalist Steven Sotloff blames US foreign policy for his death.