President Barack Obama has said the "entire world is appalled" by the murder of US journalist James Foley.
"Jim was taken from us in an act of violence that has shocked the conscience of the whole world," Obama said at a press conference.
US intelligence officials earlier confirmed that the Islamic State video of US journalists James Foley and Steven Sotloff - who is still believed to be held captive - was authentic.
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.