An image grab taken from a video published by media branch of Islamic State group on January 3, 2016, purportedly shows an English-speaking IS fighter speaking at an undisclosed location before executing five men from the Syrian city of Raqqa.

The Islamic State of Iraq and Syria (ISIS) group has threatened Britain in an Internet video on Sunday showing the killing of five “spies” it said worked with the international coalition fighting ISIS.

The SITE Intelligence Group said the video shows five men from Raqqa, capital of the self-declared extremist “caliphate” straddling Syria and Iraq, confessing to carrying out acts of espionage.

The British foreign office issued a terse statement saying only: “We are aware of the video and are examining the content.”

In the footage, an English-speaking ISIS fighter mocks British Prime Minister David Cameron for challenging ISIS, and calls him an “imbecile”.

The “confessions” of the five do not clearly identify which countries they worked with, but one mentions the international coalition led by the United States battling against ISIS in Iraq and Syria.

The video said they passed videos or photos of the living conditions in Raqqa to people in Turkey, or monitored the movements of ISIS fighters.

The five are seen wearing orange jumpsuits and kneeling before five masked fighters, all in military fatigues and armed with pistols.

“This is a message to David Cameron,” says one of the gunmen, speaking in a British accent.

British warplanes, which were already attacking ISIS positions in Iraq, at the beginning of December began targeting the group in Syria also.

The fighter says those who want to continue fighting “under the banner of Cameron” should ask themselves what would happen to them if they were to fall into ISIS hands.

He also says ISIS will wage violence and war against Britain, before the five men are killed with a bullet to the head.

ISIS often releases propaganda videos when it suffers setbacks on the battlefield, experts say.

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