Britain "will never give in to terrorism", David Cameron has told MPs in the House of Commons.
Mr Cameron condemned the "sickening and brutal" murder of US journalist Steven Sotloff by an Islamic State militant, who he said appeared to be a British citizen.
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.