President Barack Obama, in his first direct comment on the gruesome video, which shows US journalist Steven Sotloff's beheading, said that the Americans will not be "intimidated" by the "horrific act of violence" showed by the militants.
With the sharpest tone possible, he warned: "our reach is long and justice will be served".
Obama, who was speaking in a news conference in Estonia earlier on Wednesday, said that video showing the cold blooded murder of the second US scribe, which was released on Tuesday 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".
His comments came as the US National Security Council said that US intelligence agents had "analysed the recently released video showing US citizen Steven Sotloff and has reached the judgement that it is authentic". This also comes as the UK, separately, held a meeting of its emergency Cobra committee after threats to kill a British hostage who was shown at the end of the video.
'They Have Already Failed'
Obama added that IS had failed in achieving anything, as Americans are repulsed by their 'barbarism"
"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."