Britain's Prime Minister David Cameron speaks during a news conference at the Government Office in Hanoi in this July 29, 2015 file photo.[Photo/Agencies]

LONDON - British Prime Minister David Cameron on Monday condemned a new execution video by Islamic State (IS) as "desperate stuff" from an organization that is "losing anybody's sympathy."

In the video released on Sunday, a masked IS executioner with British accent threatened to invade the UK and called British Prime Minister David Cameron an "imbecile," before executing five men who appeared shackled and dressed in orange boiler suits. They were accused of spying against the IS for the UK.

"It's desperate stuff from an organization 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 on Monday.

"But this is an organization that's losing territory, it's losing ground, it's, I think, increasingly losing anybody's sympathy, and this again shows what an appalling organization we're up against," he added.

"They hate us not for what we do but for what we are - the fact that we are a successful, tolerant, democratic, multi-faith, multi-ethnic nation," the prime minister continued.

Cameron went on saying that "Britain will never be cowed by this sort of terrorism."

"Our values are so much stronger than theirs. It may take a very long time but they will be defeated," said Cameron.

The video was allegedly filmed in Raqqa, the de facto "capital" of the terrorist group in northern Syria.

"One would have thought you would have learnt the lessons from your pathetic master in Washington and his failed campaign against Islamic State," the man said in the video.

"But it seems that you, like your predecessors Blair and Brown, are just as arrogant and foolish," he said, adding: "In fact, David you are more of an imbecile."

"You will lose this war as you lost in Iraq and Afghanistan," he continued.

A boy aged about five or six, who also spoke with British accent, is also pictured in the video wearing military-style outfit.

British security and intelligence authorities have been examining the video and are working to identify the masked man, who also described the British prime minister as a "slave of the White House".

In a statement, the British Foreign Office said: "We are aware of the video and are examining its content."

In November last year, the US military launched a drone strike targeting Mohammed Emwazi, aka "Jihadi John," reportedly killing the infamous black-clad Kuwaiti-born Briton from west London. Emwazi had been pictured as a callous executioner in several beheading videos released by IS.

British warplanes have been bombing IS targets in Iraq since September 2014. British military extended its airstrikes against the group from Iraq to Syria last month.