Investigations into the identity of an English-speaking man who featured in an Islamic State (IS) group video are focusing on a Briton, Siddhartha Dhar, the BBC reported on Tuesday.

Dhar was born a Hindu and ran a business renting out bouncy castles in northeast London before converting to Islam and becoming radicalised, the broadcaster said.

It cited an unnamed official source as saying that Dhar was the main suspect, although Britain's intelligence services never comment on the record about their investigations.

On Sunday, IS released a video featuring a masked IS fighter carrying a gun and speaking English which showed the killing of five "spies" it said had worked with the international coalition fighting IS in Iraq and Syria.

Prime Minister David Cameron has described the video as "desperate stuff" designed to deflect from recent losses by IS.

In November, the US military said it was "reasonably certain" that a drone strike in Syria had killed Mohammed Emwazi, an IS fighter from London known as "Jihadi John" who had featured in a string of execution videos.

Other British media quoted the mother and sister of Dhar, 32, reportedly a member of banned group al-Muhajiroun, as saying they were not certain whether the man in the video was him.

His mother Sobita told the Daily Telegraph: "I don't know, I'm not sure of the voice. These are the most difficult questions to answer. I just cannot say."

The IS video also featured a four-year-old boy who has been identified by his grandfather as the son of woman who travelled to Syria in 2012.

British fighter jets joined the US-led coalition bombing IS targets in Syria after parliament backed the move in December.

Britain was already involved in attacking targets in Iraq.