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

Dhar was born a Hindu and ran a business renting out bouncy castles in northeast London before converting to Islam and becoming radicalized, 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, ISIS released a video featuring a masked fighter carrying a gun and speaking English which showed the killing of five "spies" it said had worked with the international coalition fighting ISIS jihadists 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" in what appears to be a British accent.

British man Henry Dare, father of Grace "Khadijah" Dare, a convert to Islam from southeast London, said he believed the boy was his four-year-old grandson Isa.  

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

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

'Human shield'

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."

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."

Dare accused his daughter of exploiting his young grandson and urged her to give up the fight.

"He's (used) for propaganda," he told Channel 4 television. "He doesn't know anything. They (ISIS) are just using him as a shield.

"She (Khadijah) should come back and face the music because she has let herself down. All of us, she's let us down."  

Grace Dare was brought up as a Christian in an ethnic Nigerian family but converted to Islam and traveled to Syria in 2012 before marrying Swedish Islamic fighter Abu Bakr, who is since believed to have been killed.

A spokeswoman for Lewisham Council, the local authority where the family grew up in southeast London, said: "We are unable to confirm the identity of the boy in the video. The council is liaising with the police and we are deeply concerned about any suggestion of a link between these abhorrent acts and our community."

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 ISIS targets in Iraq.

AFP contributed to this report