Experts are examining a video, released by Islamic State (Isis), which claims to show the beheading of second US journalist, Steven Sotloff.
In the video, a masked man carries out the decapitation of a man, believed to be Steven Sotloff, who disappeared last year near the Syrian-Turkish border while working for Time magazine.
The man in question appears on his knees while his attacker addresses the camera saying: “I’m back, Obama, and I’m back because of your arrogant foreign policy toward the Islamic State.”
The masked killer warns that the next hostage to be killed will be British.
British Prime Minister David Cameron has condemned the killing of a second American journalist by Islamic State extremists as "despicable and barbaric".
Mr Cameron has called a meeting of the Government's Cobra emergency committee for this morning.
US officials say they will try to determine as quickly as possible whether the footage is authentic, and in the meantime, President Obama's authorised extra troops be deployed to Iraq.