US says execution video of journalist Steven Sotloff is authentic

WASHINGTON - The video purporting to show United States journalist Steven Sotloff being executed is authentic, the White House said on Wednesday.

"The US Intelligence Community has analysed the recently released video showing US citizen Steven Sotloff and has reached the judgment that it is authentic," said National Security Council spokesman Caitlin Hayden.

In the footage release by the Islamic State of Iraq and Syria, Mr Sotloff calmly addresses the camera to say he is a victim of US President Barack Obama's decision to press on with air strikes in Iraq against the jihadists. The video later shows a black-clad militant cutting his throat, before displaying his severed head.

Last month another American reporter James Foley was murdered in a similar fashion.

IS also issued a threat against a British hostage warning governments to back off "this evil alliance of America against the Islamic State".

Britain's foreign minister said on Wednesday Islamic State's seizure of a British hostage did not make airstrikes on the militants more likely, but said he wasn't ruling that option out.

"It doesn't make any difference at all to our strategic planning," Britain's Foreign Secretary Philip Hammond told reporters. "If we judge that airstrikes could be beneficial ... then we will certainly consider them. But we have made no decision to do so at the moment."

Hammond was speaking after a meeting of the government's emergency response committee and said a preliminary government analysis showed the video was genuine.

He said the IS militant who appeared in the Sotloff video appeared to be the same man with an apparently British accent whose voice was featured in the video of the beheading of Foley.

"You wouldn't expect me to discuss various options that we will be considering but I can assure you that we will look at every possible option to protect this person," he added, referring to the British hostage being held by the militants.