A video posted online claims to show that Islamic State militants have killed the captured US aid worker Abdul-Rahman Kassig, the BBC reports.
In the video, a masked militant stands over a severed head which he says is that of Mr Kassig.
The US says it is working to determine the authenticity of the video, which also shows a mass killing of Syrian troops.
Mr Kassig, also known as Peter, was captured last year.
His family, who live in the US state of Indiana, said they were awaiting confirmation of the reports about their “treasured son” and had nothing more to say at this time.
If his death is confirmed, the 26-year-old will be the fifth Western hostage to have been killed by IS, following the murders of British men Alan Henning and David Haines, and US journalists James Foley and Steven Sotloff.