IS video scrutinised by UK security services
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".
IS has repeatedly used children in its propaganda videos.
In the new video, the masked jihadi 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.
Some of the five men say that they are from Raqqa in Syria while another says he is from Benghazi, Libya, but none say they are from the UK.
By Gordon Corera, BBC security correspondent
It is less than two months since British IS propagandist Mohammed Emwazi, the man known as Jihadi John, was killed by a drone strike in Syria.
This latest video by IS seems to show another man trying to take up his mantle.
The identity of the masked man is so far unknown but authorities will be trying to identify him and confirm if he is British.
Emwazi was killed after intelligence - most likely from informers - located him in a vehicle in Raqqa, Syria.
The fact that this latest video shows the killing of alleged spies indicates the extent to which IS is trying to track down those who might be providing information.
Children also regularly feature in IS propaganda videos and the brief appearance of a child - apparently with a British accent - at the end of the video will also most likely be of interest to the authorities.
BBC World Service Middle East editor Alan Johnston says the executed men appear to be "speaking under the most extraordinary duress" and "may be entirely innocent".
He also says the release of the video comes in the aftermath of a "major military setback for IS" after it recently lost control of much of the Iraqi city of Ramadi.
He adds: "It's possible this is aimed at distracting attention from that defeat - an effort to shock watching Westerners and shift their focus."
Following the death of Mohammed Emwazi, the Briton who became known as Jihadi John, this new video shows "something of a replacement" for him, says our correspondent.
"That masked militant becomes the new masked face of the Islamic State, at least for British viewers," he says.
Jonathan Russell, head of policy at the Quilliam Foundation think tank, said it was "shocking" to hear British voices and to see a child in the video, but that both were being used "to reinforce the IS brand".
He also said such actions were "projecting falsehoods" and did not "offer anything to potential recruits here in the West".