Home Secretary Theresa May has described Islamic State as a "group of murderous psychopaths".
Reacting to news of US journalist Steven Sotloff's murder, Mrs May said it was another example of a "barbaric act" by the militant group.
May said her thoughts were with the victim's family.
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.