British Prime Minister David Cameron has condemned the Islamic State group as a "depraved organization" after the extremists released a video claiming to show that militants have beheaded U.S. aid worker Peter Kassig.

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 Sunday 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.

U.S. officials said they were working to determine the video's authenticity.