An unsuccessful attempt was made to rescue a British hostage being held by Islamic State, the Foreign Secretary said.
"A rescue attempt took place some time ago - unfortunately unsuccessfully," Philip Hammond said after an emergency Cobra meeting.
Mr Hammond said the government had been aware of a British citizen being held by militants for some time.
He said all possible options were being considered to protect the hostage.
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.