US 'appalled' by execution of journalist James Foley

US intelligence was attempting Tuesday to authenticate a video purporting to show an Islamic State militant killing a US journalist, the White House said, adding that it would be "appalling" if genuine.

"We have seen a video that purports to be the murder of US citizen James Foley by ISIL," deputy National Security Council spokeswoman Caitlin Hayden said, referring to the 40-year-old reporter.