The video footage released on Sunday bears the markings of the "Islamic State" (IS) media wing and shows five men "confessing" to filming and photographing sites in Raqqa.

The apparent prisoners, speaking in Arabic, are introduced as "the enemy." Later, clad in orange jumpsuits, they are forced to kneel and line up in the desert, where they are shot by masked executioners.

The propaganda video could not be independently verified.

Before the shooting, an English-speaking masked fighter mocks British Prime Minister David Cameron for challenging IS and calls the men "spies."

He threatens that IS will attack Great Britain: "We will continue to wage jihad ... and one day invade your land, where we will rule by the sharia."

Experts underlined that IS often released propaganda videos when it suffered setbacks on the battlefield.

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

On December 26, an alliance of Kurdish and Arab fighters in northern Syria seized the Tishreen Dam from IS. The jihadist group had held the dam since 2014.

das/rc (AFP, AP)