A video posted online claims to show that Islamic State militants have killed the captured US aid worker Abdul-Rahman Kassig, the BBC reports on Sunday.
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.
