By ITV News Washington Correspondent Robert Moore
With an American citizen butchered on camera for the world to see and President Obama personally being taunted, I think it may be politically, even emotionally, impossible for the US not act more aggressively against the Islamic State.
Given that the IS effectively no longer acknowledges the Iraqi-Syrian border, one option is for US warplanes to strike now deep inside Syria. But there are clearly some very major problems with that. Any such intervention in Syria would be be seen as siding to strengthen President Assad, and strengthening in many ways Shiite forces.
So that would have unforeseen consequences. In addition, any realistically major campaign against the Islamic State is going to need the deployment of US Special Forces, and that in turn would break this President's single most significant pledge to the American people - that there would not be boots on the ground. I think for the President and for his US security team, this remains an almost impossible dilemma.
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.