President Barack Obama has stressed that Islamic State "speaks for no religion".
"No just God would stand for what they did yesterday and what they do every single day," he said in response to the video released showing the beheading of US journalist James Foley.
"Isil has no ideology of any value to any human beings their ideology is bankrupt," Obama said. "Isil has no place in the 21st Century."
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.