Investigations into the identity of an English-speaking man who featured in a Daesh 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.

A spokesman for Prime Minister David Cameron said on Tuesday that "work is underway to examine the content and the individuals in that video", but that it was unlikely the authorities would publicly confirm the man's identity at any stage.

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

It also features a young boy wearing camouflage clothing and black headband, saying "We are going to go kill the kuffar (non-believers) over there".

The boy was identified by his grandfather as the son of a British woman who travelled to Syria in 2012.

The grandfather accused Daesh of using the boy, who is reported to be four-years old, as a "shield".

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

In November, the US military said it was "reasonably certain" that a drone strike in Syria had killed Mohammed Emwazi, a Daesh 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.

"From hearing the audio, it did sound how I remembered hearing my brother... but in terms of physique... I'm not convinced it is him," said his sister Konika Dhar.

"If he has (appeared in this video), I'm going to go there and kill him myself," she added.

"I cried and I was like, 'oh my God what is he doing?' I'm just going to strangle him when I see him, but this is assuming it is him.

"All I can say is I still really miss him, he's still my brother."

According to British media reports, Dhar travelled to Syria after catching a bus to Paris with his family, despite being on bail for alleged terrorism offences.

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

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

Britain was already involved in attacking Daesh targets in Iraq.