Latest ISIS video features child with British accent threatening to kill non-believers
An Islamic State video featuring a masked British extremist and a young boy is being examined by security services.
The footage released online shows the brutal murders of five men accused of spying for the UK.
A jihadi is seen waving a gun as he mocks Prime Minister David Cameron and the RAF bombing campaign against IS in Iraq and Syria.
Five men in orange jumpsuits "confess" to filming and photographing sites in exchange for money within Raqqa, the capital of IS's self-declared caliphate.
The militant claimed the Government had "abandoned these spies" as the men are seen kneeling before they are shot in the head.
Following the killings the 10-minute clip ends with a little boy dressed in military fatigues and wearing a black headband.
Speaking with what sounds like a British accent the child points into the distance and says in English: "We are going to go kill the kaffir (non-believers) over there."
IS - also known as Daesh, Isil and Isis - has used children in a number of previous propaganda videos.
The latest film bears similarities to those which featured the British jihadist known as Jihadi John.
The killer, real name Mohammed Emwazi, appeared in videos showing the murders of Britons David Haines and Alan Henning, plus those of US journalists James Foley and Steven Sotloff.
Emwazi was killed in a US drone strike in Syria in November.
It comes days after Mr Cameron promised to crack down on IS sympathisers, stressing in a New Year's message that all Britons should have "loyalty" to their country.
Pointing a handgun, the masked militant addresses the Prime Minister, saying: "How strange it is that a leader of a small island threatens us with a handful of planes. One would have thought you would have learned the lessons of your pathetic master in Washington and his failed campaign against the Islamic State.
"But it seems like you, just like your predecessors (Tony) Blair and (Gordon) Brown are just as arrogant and foolish. In fact David, you (waves handgun) are more of an imbecile. Only an imbecile would dare to wage war against a land where the law of Allah reigns supreme and where the people live under the justice and security of the sharia.
"Only an imbecile would dare to anger a people who love death the way you love your life."
The jihadi goes on to say that IS is "here to stay" and will invade the UK and rule under sharia law.
He adds: "As for those of you who wish to continue fighting under the banner of Cameron, on the minimum wage, we say to you, to ask yourself, 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? Because you will lose this war, as you lost in Iraq and Afghanistan."
The five victims' identities have not been verified but they have been named in reports as Ha'il Marwan Abdul Razaq, 40, Ubi Muhammad Abdul Ghani, 26, Faisal Hamud al-Ja'far, 25, Umaar Hamud al-Ja'far, 30, all from Syria, and Mahyar Mahmud al-Uthmaan, 31, whose origin is unclear.
Whitehall sources suggested the video was an attempt to distract attention from recent military setbacks IS has faced, like losing the Iraqi town of Ramadi to state forces.
A Foreign Office spokesman said: "We are aware of the video and are examining its content."
A member of a Raqqa-based group which opposes IS, named by the BBC as Abu al-Furat, told Radio 4's Today programme that the victims included shopkeepers and businessmen from the town. IS claimed that one of them had been using his Internet cafe for spying purposes, he said.
"I know personally two of them, who started demonstrating very early," he said. "They are the sons of the Syrian revolution in Raqqa.
"Daesh is collapsing at the moment, but they are trying by doing this to say that their security bodies are working very well and they are still strong."
He added: "Daesh is trying to go with the events and developments in the region. Our own information says that they turned mad after the British forces started striking their sites and that led to the death of some of their leaders - one of them was John - and Daesh after that tried to crack down on the media people and the activists.
"This group that they have killed, we doubt that they are spying for Britain. Daesh has been harmed very much by the death of some British citizens fighting for Daesh. In this video, the executor is trying to copy John. They want to say that if John died, there are others that can do this job."
Dr Alan Mendoza, executive director of think tank The Henry Jackson Society, said: "This new video shows that Islamic State promises to start 2016 as it ended 2015: posing a serious threat to our national security interests.
"Their statement of intent and renewed ability to recruit jihadists from the UK must be met with a strong show of action from our leaders.
"Abroad we will need to step up the campaign against IS to ensure it is defeated, while at home, further safeguards against radicalisation need to be introduced to protect those vulnerable to IS propaganda from joining their forces."