The Islamic State group threatened Britain in an Internet video Sunday showing the killing of five "spies" it said worked with the international coalition fighting IS in Iraq and Syria.

The SITE Intelligence Group said the video shows five men from Raqa, capital of the self-declared jihadist "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 IS fighter mocks British Prime Minister David Cameron for challenging IS, 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 IS in Iraq and Syria.

The video said they passed videos or photos of the living conditions in Raqa to people in Turkey, or monitored the movements of IS 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 IS positions in Iraq, at the beginning of December began targeting the jihadists 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 IS hands.

He also says IS will wage jihad against Britain, before the five men are killed with a bullet to the head.

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

In Iraq last Sunday, federal forces declared victory in the battle for Ramadi, which IS seized in May 2015.

And on December 26, an alliance of Kurdish and Arab fighters in northern Syria seized the strategic Tishreen Dam from IS which the jihadist group had held since 2014.