Abdul-Rahman Kassig was 'an incredible person' - friend

President Barack Obama has confirmed the death of US aid worker Abdul-Rahman Kassig, whose beheading was shown in a video posted by Islamic State (IS).

The footage, authenticated by the White House, shows a masked man standing over Mr Kassig's severed head. It also shows the beheading of 18 Syrian captives.

The 26-year-old is the fifth Western hostage to be killed by IS, which controls large parts of Syria and Iraq.

His friend, journalist Michael Downey, described him as "an incredible person".