BEIRUT -- A video circulated online Sunday purported to show the Islamic State of Iraq and Syria (ISIS) killing five men accused of spying for Britain in Syria.

The high-quality footage bore the markings of the ISIS media wing, and shows five men "confessing" to filming and photographing sites in exchange for money within Raqqa, the capital of the group's self-declared caliphate. It could not be independently verified.

The men, speaking Arabic, are introduced as "the enemy" and "apostates." Several said they were given hidden cameras with lenses disguised as shirt buttons to provide images and videos to a contact in Turkey. One said he later saw that his videos appearing in international media including Britain's BBC.

He goes on to say he was sent two photos of British ISIS members and asked to locate them, and also to photograph a former Syrian government building now housing ISIS's "Islamic Courts."

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"Don't let them fool you like they fooled us," one of the condemned men said, referring to those seeking to collect information on ISIS and undermine it.

Another man appeals to people in similar situations to confess and turn themselves in, repeatedly saying "the door to repentance is open."

The video then cuts to the men kneeling, lined up in orange jumpsuits in the desert, where they are shot in the head by masked men.

Before the shooting, a masked man with a British accent mocks British Prime Minister David Cameron, calling him a "slave of the White House," and "mule of the Jews." He describes the men as "spies," and says ISIS will one day invade Britain and impose their extremist version of Islamic law.

"Only an imbecile would dare to wage war against a land where the law of Allah reigns supreme," he says, referring to Cameron. "The Isla mic State, our country, is here to stay. And we will continue to wage jihad, break borders, and one day invade your land, where we will rule by the Shariah."

Former Deputy CIA Director Michael Morell said Sunday on CBS' "Face the Nation" that the timing of the video is likely not a coincidence, as the Iraqi Army recently retook the city of Ramadi from ISIS. The provincial capital of the Anbar province fell to ISIS in May.

"This is a strategically significant city," Morell said, pointing to its relative proximity to Baghdad and spots on the roads to Syria and Jordan. He added that it was important that Iraqi security forces won the fight, since other gains against ISIS have been won by the Kurdish army or Shia militias.

Morell said the victory in Ramadi highlights the relative success for U.S.-backed forces in Iraq versus almost no success in Syria.

ISIS has come under pressure in Iraq and Syria. In addition to the loss in Ramadi, ISIS lost the town of Sinjar in Iraq in November, and areas across the border in Syria at the same time.