U.S. intelligence officials have authenticated a video released Tuesday showing ISIS extremists beheading American freelance journalist Steven Sotloff. The beheading came two weeks after the murder of journalist James Foley by the same group.

Sotfloff went missing in Syria in 2013 while reporting for Time and Foreign Policy Magazines. He appeared in a video last month in which Foley was beheaded. In that video, an ISIS representative threatened to behead Sotloff if President Obama did not order an end to airstrikes against ISIS forces in Iraq.

Obama addressed the latest video Wednesday while traveling in Estonia ahead of a NATO summit in Wales, saying the United States will not be intimidated by acts of violence.

"Their horrific acts only unite us as a country and stiffen our resolve to take the fight against these terrorists," he said. "And those who make the mistake of harming Americans will learn that we will not forget, and that our reach is long and that justice will be served."

 Meanwhile, British officials have confirmed the identity of a British national being held by the group, which has threatened to behead him if airstrikes are not halted. David Cawthorne Haines, formerly a consultant director for Croatia-based manufacturing company Astraea, went missing in Syria in 2013 while working with an aid group.

British Prime Minister David Cameron has expressed anger and disgust at ISIS' actions, and has said that his government "will not be cowed by these barbaric killers."

"What has happened to the two hostages so far and what may happen again in the future is utterly abhorrent and barbaric and these people need to understand we will not waver in our aim of defeating terrorism," he said during a recent television appearance.

Cameron has not said that Britain would participate in retaliatory airstrikes, but some in his government are pressuring him to do so. Meanwhile, the White House has yet to announce any plans for a response to this latest murder of an American by ISIS.