LONDON - The murder of David Haines was an "act of pure evil", David Cameron has said after the release of a video appearing to show the UK hostage's beheading.
The 44-year-old aid worker was seized in Syria in 2013. He was being held by Islamic State militants who have already killed two US captives.
The latest video also includes a threat to kill a second British hostage.
The PM vowed to do everything possible to find the killers. Mr Haines's family said he would be "missed terribly".
Born in Holderness, East Yorkshire, Mr Haines went to school in Perth and had been living in Croatia with his second wife, who is Croatian, and their four-year-old daughter. His parents live in Ayr.
In a statement released by the Foreign Office, Mike Haines said his brother, a father of two, "was and is loved by all his family".
"David was most alive and enthusiastic in his humanitarian roles. His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair," he said.