John Kerry has urged America's "core coalition" including the UK to contribute to the campaign against Islamic State (IS).
But the US Secretary of State insisted there will be "no boots on the ground".
Mr Kerry called for a plan to counter the threat of IS in time for the UN General Assembly later this month.
David Cameron has repeatedly refused to rule out joining the US in carrying out air strikes on IS strongholds.
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.