British security authorities examining new IS execution video

British security and intelligence authorities are examining a new video released by Islamic State (IS) threatening to invade the UK, local media reported Monday.

In the video released on Sunday, a masked IS executioner with British accent called British Prime Minister David Cameron an "imbecile" before executing five men who appeared shackled and dressed in orange boiler suits. They were accused of spying against the IS for the UK.

The video was allegedly filmed in Raqqa, the de facto "capital" of the terrorist group in northern Syria.

British authorities are working to identify the masked man, who described the British prime minister as a "slave of the White House."

"One would have thought you would have learnt the lessons from your pathetic master in Washington and his failed campaign against Islamic State," the man said in the video.

"But it seems that you, like your predecessors Blair and Brown, are just as arrogant and foolish," he said, adding: "In fact, David you are more of an imbecile."

"You will lose this war as you lost in Iraq and Afghanistan," he continued.

A boy aged about five or six, who also spoke with British accent, is also pictured in the video wearing military-style outfit.

In a statement, the British Foreign Office said: "We are aware of the video and are examining its content."

British warplanes have been bombing IS targets in Iraq since September 2014. The country extended its airstrikes against the group from Iraq to Syria last month. Endit