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.
In it, 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 following sequence shows the five wearing orange jumpsuits and kneeling before five masked fighters, all in military fatigues and armed with pistols.
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"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.
"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?"
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.