And, for that lack of discipline, the group was sorry. Killed: Steven Sotloff. Credit:Reuters "We tried to remove the video after we understood that this was published by mistake, and we are sorry to the followers of Islamic State," the group said in a message, in Arabic, posted on Justpaste. Vocativ, a New York-based company that says it uses "deep-web technology" to report the news, said chatter in IS forums suggested the initial video release was an unintentional leak from the militant group. The website reported that an IS-affiliated Twitter account, which has since been suspended, first published the video, before another IS-affiliated account, @Khattabyaz pointed out the error.

"A clarification about the mistake was made by 'Uyun al-Ummah' account, that has published the video before the official time," IS said on Justpaste. "The user saw a tweet with the video and thought it was published officially. We tried to remove the video after we understood that his was published by mistake, and we are sorry to the followers of the Islamic State." In the video, Sotloff says he is "paying the price" for the US government's decision to strike IS targets in Iraq. A masked fighter with a British accent, who is believed to have appeared in the video of American Journalist James Foley's beheading on August 19, also stands beside Sotloff. He says: "I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State."

Some Western intelligence officials have said they believed Sotloff may have been executed on the same day as Foley, and that the propagandists at IS had decided to space out the publicity of each one. However, the most recent video shows Sotloff with a small beard and some hair on his head, in contrast to the August 19 video, in which he is shown as nearly bald and clean shaven. The contrast in his appearance suggests the videos were not taken at the same time. Sotloff's family issued a statement via a spokesman, Barak Barfi, that suggested they believed the video was authentic. Loading "The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time," Mr Barfi said.

The SITE Intelligence Group, a research organisation that tracks jihadist web postings, said IS was threatening to behead a third captive, British man.