The UK's security and intelligence agencies are examining a grisly Islamic State video to identify a man and young boy with British accents, threatening Prime Minister David Cameron that the terror group will one day "invade" Britain and "rule by the Sharia".
The 10-minute 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 and goes on to say that the video, which has not been independently verified, is a message for Prime Minister 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 video, the masked man who has a British accent and is holding a gun, mocks Cameron for daring to "challenge the might" of the extremist group.
He goes on to say: "We will continue to wage jihad, break borders and one day invade your land where we will rule by the Sharia."
The five men, wearing jumpsuits and kneeling in a desert location, then appear to be shot in the back of the head, 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 say they are from Raqqa in Syria while another says he is from Benghazi, Libya, but none say they are from the UK.
A UK Foreign Office spokesman said: "We are aware of the video and are examining its content."
IS, also referred to as Daesh, has previously released videos showing killings, including footage showing the apparent beheading of two US journalists, James Foley and Steven Sotloff, and two British aid workers David Haines and Alan Henning.
Mohammed Emwazi, the Briton who became known as Jihadi John, appeared in the videos. He was killed in a US drone strike in Syria in November.
Britain last month carried out its first airstrikes in Syria, hours after MPs voted overwhelmingly to authorise military action in the global fight against ISIS.
The IS group, notorious for its brutality, seized large swathes of territory in Syria and Iraq in 2014, when it formally declared the establishment of a "caliphate" - a state governed in accordance with Islamic law, or Sharia, by God's deputy on Earth, or caliph.
