President Barack Obama has vowed the US will not be intimidated, after Islamic State militants released a video showing the beheading of American journalist Steven Sotloff.
Mr. Obama warned: “Our reach is long and justice will be served.”
Another US journalist, James Foley, was similarly murdered last month.
Separately, the UK held a meeting of its emergency Cobra committee after threats to kill a British hostage who was also shown in the latest video.
US National Security Council spokesperson Caitlin Hayden said that US intelligence agents had “analysed the recently released video showing US citizen Steven Sotloff and has reached the judgment that it is authentic.”
Speaking in Estonia, Mr. Obama said the beheading was a “horrific act of violence and we cannot begin to imagine the agony everyone who loves Steven is feeling right now. Our country grieves with them.”
He added: “Whatever these murderers think they will achieve by killing innocent Americans like Steven, they have already failed.
“They have failed because, like people round the world, Americans are repulsed by their barbarism. We will not be intimidated. Their horrific acts only unite us as a country and stiffen our resolve.
“Those who make the mistake of harming Americans will learn that we will not forget, that our reach is long and that justice will be served.”