The group calling itself Islamic State has beheaded the American hostage Peter Kassig, according to claims in a video posted online.
The film, which euronews is not showing, features an apparent execution carried out by a masked man.
The 26-year-old aid worker, a former soldier who had changed his name to Abdul Rahman, devoted his time to helping Palestinian and Syrian refugees.
He had written to his parents – who had pleaded directly to his captors to release him – saying if he did die, people should take comfort in the fact he had worked to alleviate suffering.
Kassig was working for his own charity SERA – Special Emergency Response and Assistance – when he was captured in October last year.
It has stopped operating in Syria because of the security situation.
Washington says if the video is authentic, the White House would be “appalled by the brutal murder of an innocent American”.
British Prime Minister David Cameron said the apparent beheading again showed ISIL to be a “depraved organisation”.
Peter Kassig had appeared in an ISIL video showing the murder of British worker Alan Henning, which was roundly condemned by Muslims.