A second journalist is seen at the end of a video appearing to show the beheading of James Foley by Islamic State militants.
The US reporter, Steven Joel Sotloff, who went missing near the border of Syria and Turkey last year, is threatened at the end of the footage as he kneels in an orange jumpsuit.
The Islamic State fighter, speaking with a British accent, says Sotloff's life depends on President Obama's "next decision".
Sotloff had written for TIME, The National Interest and Media Line before his disappearance in August 2013.
A British family have told ITV News of their devastation after discovering a close relative is fighting with the Islamic State in Iraq.
A French journalist who was held hostage with James Foley in Syria has told ITV News it was "devastating" to see his murder.