President Barack Obama said all Americans "feel the ache" of the loss of James Foley, the US journalist beheaded in a video released by Islamic State militants.
Obama said Foley was "a man who lived for his work, who courageously told the stories of his fellow human beings and was liked and loved by friends and family."
"All of us mourn his loss," the President said.
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.