VIDEO: ISIS threatens to invade Britain in new execution video

Damascus: The Islamic State (IS) group threatened to invade Britain in a new video on Sunday, according to the Syrian Observatory for Human Rights.

In the video, an unnamed 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 Britain.

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

Last month, Cameron said Britain has a moral and military duty to attack the IS in Syria, while the British military has already extended an air campaign against the group from Iraq to Syria.