He also threatened to execute another hostage, Briton David Haines, and called on the government to stop supporting "diabolical alliance of America against the Islamic state".
Freelance journalist Stephen Sotloff was kidnapped by islamists crossing the Syrian border in August 2013.
The video of execution of another American journalist - James Foley was posted on internet on August 19, 2014. Fighters of the "Islamic state" beheaded him too.