BEIRUT, Oct 3 (Reuters) - Islamic State militants fighting in Iraq and Syria released a video on Friday which purported to show the beheading of a man identified as British citizen Alan Henning. Reuters could **t immediately verify the footage on YouTube, to which pro-Islamic State Twitter feeds provided links. Two U.S. journalists, James Foley and Steven Sotloff, have previously been executed by Islamic State, as well as a British aid worker, David Haines. (Writing by Sylvia Westall; Editing by Mark Trevelyan)