Speaking in a broadcast message after the release on Friday (local time) of a video showing the decapitated body of British aid worker Alan Henning, Cameron said the whole country was mourning.
He added: "In terms of what we will do, we will use all the assets we have ... to try and find these hostages, to try and help these hostages ... and do everything we can to defeat this organisation which is utterly ruthless, senseless and barbaric in the way it treats people."
He was speaking after meeting the heads of Britain's armed forces and intelligence agencies at his country residence, Chequers.
Henning had been part of an aid convoy taking medical supplies to a hospital in northwest Syria in December last year when it was stopped by gunmen and he was abducted.
Cameron said no appeals to spare the life of the 47-year-old taxi driver from Salford in northern England had made any difference.
"The fact that this was a kind, gentle, compassionate and caring man who had simply gone to help others -- the fact they could murder him the way they did shows what we are dealing with," he added.
"With others, we must do everything we can to defeat this organisation."
