American journalist Steven Sotloff beheaded by Islamic State - reports

The so-called Islamic State (IS) has released a video in which they claim to have beheaded American journalist Steven Sotloff, according to an organisation that monitors the group.

The SITE Intelligence Group reported on Tuesday afternoon that the IS has released a 3 minute long video in which they show the beheading of Sotloff and threaten to execute another captive.

The video has clear political message as in it IS restate their demand for American air strikes to end against the group in Syria and Iraq, warning of further executions should the bombing campaign continue.

Sotloff appeared in a video that purported to show the execution of fellow American journalist James Foley. In it, a British accented man wielding a knife said Foley was being executed because of the American's air strikes against the group in Iraq.

Early reports suggest the same British voice is heard in the latest IS video.

Sotloff, 31, disappeared in Syria while reporting during August last year. His mother recently released a plea to the IS asking for her son to be returned home safely.

A spokesperson for the Soltoff family told American media on Tuesday they are “aware of the video” but awaiting confirmation. The spokesperson said the family are “grieving and want their privacy.”

The White House has responded by saying they cannot confirm the authenticity of the video at the moment.

"I'm not in a position to confirm the authenticity of that video or the reports," said White House spokesperson Josh Earnest.

"If there is a video that's been released, it's something that will be analysed very carefully by the US government and our intelligence officials to establish its authenticity."

An American official said the Obama administration was formulating a plan to combat the IS.

“We are putting the features in place, developing a broad regional coalition, a broad international coalition, working to get a new Iraqi government stood up, working to get our plans in place,” Brett McGurk, Deputy US Assistant Secretary of State for Iraq and Iran, told CNN’s Amanpour.

“So stay tuned.”

McGurk said the IS “is a sophisticated organisation. You cannot just go in militarily and start dropping bombs and hope that it’s going to work out. You have to have a very sophisticated approach to this.”

British Prime Minister David Cameron described the news as “absolutely disgusting and despicable” and said he will make a statement on the incident “later.”

The Committee to Protect Journalists condemned the apparent execution and called on those responsible to be held to account.

“Journalists know that covering war is inherently dangerous and that they could get killed in crossfire,” said CPJ Executive Director Joel Simon. “But being butchered in front of camera simply for being a reporter is pure barbarism. He [Steven Sotloff], like James Foley, went to Syria to tell a story.”

“They were civilians, not representatives of any government. Their murders are war crimes and those who committed them must be brought to justice swiftly,” he added.