British authorities examining new IS execution video

British security and intelligence authorities are examining a new video released by the Islamic State (IS) terror group threatening to invade the country, the 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 suits, Xinhua news agency reported.

They were accused of spying against the IS for Britain.

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 learned 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 a British accent, is also pictured in the video wearing military-style outfit.

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