The intelligence community is working to determine if a video purportedly showing the killing of US journalist Steven Sotloff by Islamic State militants is authentic, State Department spokesperson Jen Psaki has told a White House press briefing.
If the video is genuine we are sickened by this brutal act taking the life of another innocent American citizen; our hearts go out to the Sotloff family and we will provide more information as it becomes available.
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.