London: The UK's security and intelligence agencies are examining a video by so-called Islamic State, to identify a man and young boy with British accents.

The video purportedly shows the killing of five men who IS says were spying for the UK.

The authorities will also be carrying out voice-print analysis to match what is heard against individuals known to have travelled to Iraq and Syria.

The man threatens attacks in the UK, during the 10-minute film.

He also says the footage, which has not been independently verified, is a message for Prime Minister David Cameron.

After the apparent killings the young boy, who seems to be aged about six or seven years old and is wearing military-style clothing, is seen pointing into the distance and talking about killing "unbelievers".

In the new video, the masked militant, who is holding a gun, mocks Mr Cameron for daring to "challenge the might" of the extremist group before he makes a threat to British people to "invade your land".

The five men, wearing jumpsuits, are seen purportedly being shot in the back of the head in a desert location, after making what is claimed to be their confessions.

One of the men says he had been asked to provide information about the location of IS militants, including two Britons, apparently to help target them with air strikes.