Updated | US authorities confirm beheading video is authentic

US authorities have confirmed that a video released yesterday by the Islamic State in Iraq and Syria is authentic.

ISIS has beheaded Steven J. Sotloff, the second American executed by the Islamic militant group, and posted a video of it on the Internet. A research organisation that tracks jihadist web postings, the SITE Intelligence Group, revealed the video.

Sotloff's family issued a statement saying it believed he had been killed.

The execution of Sotloff, 31, came despite pleas from his mother aimed directly at ISIS’s top leader seeking mercy for her son, a freelance journalist who was captured in northern Syria a year ago.

Word of Sotloff’s beheading came two weeks after James Foley, 40, another American journalist, was beheaded by ISIS, which warned that Sotloff would be the next to die.

The videotaped beheadings and threats by ISIS have helped rocket the group’s ascendance into a top crisis for the Obama administration and its Western allies.

Sotloff’s family members issued a statement that suggested they believed the video was authentic: “The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time.”

Josh Earnest, a White House spokesman, said he could not immediately confirm the authenticity of the video showing Sotloff’s killing. “It is something that will be analyzed very carefully by the U.S. government and our intelligence officials to determine its authenticity,” he told reporters in Washington.