British Prime Minister David Cameron on Monday described an Islamic State group video apparently showing the execution of five British "spies" as "desperate stuff" designed to deflect from their recent military losses.

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

"It's desperate stuff from an organisation that really does do the most utterly despicable and ghastly acts and people can see that again today," Cameron said during a visit to east London.

"This is an organisation that's losing territory... increasingly losing anybody's sympathy, and this again shows what an appalling organisation we're up against."

In the footage, an English-speaking IS fighter mocks Cameron for challenging IS, and calls him an "imbecile".

"This is a message to David Cameron," says one of the gunmen, speaking in a British accent.

Britain's Foreign Office issued a short statement saying: "We are aware of the video and are examining the content."

The prime minister said IS was motivated by a hatred for Britain's "tolerant, democratic, multi-faith" values and that the jihadists would not succeed in the long term.

"Britain will never be cowed by this sort of terrorism. Our values are so much stronger than theirs," he said.

"It may take a very long time but they will be defeated."

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

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Cameron International