Cameron said that like others who have been "murdered in cold blood" by the group, Kassig was a selfless humanitarian worker who had gone to the region to help others.
Cameron said today the video "underlines the deplorable depths to which these terrorists are prepared to go -- savagely murdering a compassionate man."
The video, which also purports to show extremists beheading a dozen Syrian soldiers, ends with a militant standing over what he calls the severed head of Kassig.