A video has been released online by so-called Islamic State, which it says shows the killing of five men who it claimed had been spying for the UK.
A masked militant speaking with a British accent appears in the 10-minute film, threatening attacks in the UK.
He says this is a message for Prime Minister David Cameron.
A young boy is also seen speaking in English and talking about killing unbelievers. The propaganda video has not been independently verified.
In the video, some of the men say that they are from Raqqa in Syria and another says he is from Benghazi, Libya, but none say they are from the UK.
BBC World Service Middle East editor Alan Johnston said the video shows five men making what is claimed to be their confessions but they appear to be “speaking under the most extraordinary duress”.
“They may be entirely innocent,” he added.
One says he had been asked to provide information about the location of IS militants, possibly so that they could be targeted by air strikes, our correspondent said.