DUBAI (Reuters) - The Islamic State in Iraq and Syria (ISIS) militant group has released a video purporting to show the beheading of U.S. hostage Steven Sotloff, the SITE monitoring service reported on Tuesday.

A masked figure in the video also issued a threat against a British hostage, a man the group named as David Haines, and warned governments to back off "this evil alliance of America against the Islamic State", the monitoring service said.

Sotloff, a freelance journalist, was kidnapped in Syria in August 2013. Sotloff's mother Shirley appealed on Aug. 27 in a videotaped message to Islamic State's self-proclaimed caliph, Abu Bakr al-Baghdadi, for her son's release.

On Aug 19 the group published a video showing the beheading of U.S. hostage James Foley. The group said his death was in retaliation for U.S. airstrikes on fighters in Iraq.

That video was issued after the United States resumed air strikes in Iraq in August for the first time since the end of the U.S. occupation in 2011.