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 following sequence shows the five 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.

"How strange it is that we find ourselves today hearing an insignificant leader like you challenge the might of the Islamic State."

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

"Only an imbecile would dare to wage war against a land where the law of Allah reigns supreme and where the people live under the justice and security of sharia" or Islamic law, the fighter says.

"We will continue to wage jihad... and one day invade your land, where we will rule by the sharia."

He said those who want to continue fighting "under the banner of Cameron" should ask themselves: "Do you really think your government will care about you when you come into our hands, or will they abandon you as they have abandoned these spies and those who came before them?"

The five men are then 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.