Jihadists have released a video apparently showing the beheading of a journalist from the United States kidnapped in Syria, in the most direct retaliation yet to nearly two weeks of US airstrikes in Iraq.

As calls mounted for Washington to expand its military intervention against the Islamic State jihadists, the group threatened to kill another US reporter if the strikes did not stop.

The video, posted online on Tuesday, showed a masked militant beheading a man resembling James Foley, who has been missing since being seized in Syria in November 2012.