The White House said that it had yet to authenticate the footage, which SITE said showed 31-year-old freelance writer Steven Sotloff on his knees beside an ISIL terrorist with a knife.
The masked militant condemns US attacks against ISIL positions and cuts Sotloff's throat. He then introduces a second captive, identified by name and said to be British.
"I'm back, Obama, and I'm back because of your arrogant foreign policy towards the (so-called) Islamic State," the militant says, in apparent reference to a previous video in which US journalist James Foley was murdered.
White House spokesman Josh Earnest said he had not been aware of the report before taking the podium at his daily briefing, but added that the administration's "thoughts and prayers" were with Sotloff's family.
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