The sister of a British jihadi has said she will "kill him myself" if he is revealed as the masked militant in a new Islamic State video showing the murder of five hostages.

Konika Dhar, from north London, said she does not believe her brother is the man in the film, but she conceded the voice sounded "a bit like him".

It came as a small boy who appeared in the footage was named in reports as the son of a jihadi bride from south-east London.

The identities of those in the ten-minute clip, which is being closely examined by security services, have not been confirmed.

British Muslim convert Abu Rumaysah previously talked about the UK being brought under sharia law by a conquering Islamic State (IS) army.

Born a Hindu called Siddhartha Dhar, he ran a business renting out bouncy castles in London before converting.

He was arrested in September 2014 when he was 31, with eight other men as part of an investigation into alleged support of the banned extremist group al-Muhajiroun.

But in a huge embarrassment for authorities he fled Britain with his family the following day after being released on bail, reportedly travelling to Paris and then Syria.

Speaking today Ms Dhar said: " I don't think it is, personally, to be honest." She went on: "I examined the features - one eye is smaller than the other, my brother has symmetrical eyes.

"The eyebrows are bushy and this guy is taller, my brother is shorter and he has got broader shoulders, but he has got stooped shoulders so I don't think it is.

"I think the voice sounds a bit like him, but it could be someone else, but he could have helped with a voiceover, but I have no idea."

She said she has not had contact with her brother in more than a year, saying she "doesn't even know who he is" any more.

Ms Dhar said: "If it is him, bloody hell am I shocked? I am going to kill him myself. He is going to come back and I am going to kill him if he has done this.

"I can't believe it. This is just so shocking for me. I don't know what the authorities are doing to confirm the identity, but I need to know if it is."

Meanwhile, reports claimed a young child seen at the end of the video dressed in military-style clothes and wearing a black headscarf with an IS logo on it was Isa Dare, the son of Grace "Khadija" Dare from Lewisham, south-east London.

The boy points into the distance and says in English: "We are going to go kill the kaffir (non-believers) over there."

Grace Dare was brought up as a Christian but converted to Islam and started using the first name Khadijah. She travelled to Syria in 2012 and married a Swedish man called Abu Bakr, who is believed to have been killed since.

The extremist, who according to the Daily Telegraph has a younger son as well as Isa, previously took to social media to say she wanted to be the first British woman to kill a British or American ISIS hostage. Last year she was said to have posted a picture on Twitter of Isa with an AK-47 rifle.

A spokeswoman for Lewisham Council 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."

David Cameron described the video as "desperate stuff" from a group that is "losing territory".

He said he had viewed part of the video, adding: " I think it is absolutely desperate tactics from an organisation that is clearly now on the back foot," said the Prime Minister.

The footage shows f ive men in orange jumpsuits "confessing" to taking money in return for filming and photographing sites within Raqqa, the capital of IS's self-declared caliphate.

The militant claims the UK Government had "abandoned these spies" as the men are seen kneeling before being shot in the head.

The five victims have been named in reports as Ha'il Marwan Abdul Razaq, 40, Ubi Muhammad Abdul Ghani, 26, Faisal Hamud al-Ja'far, 25, Umaar Hamud al-Ja'far, 30, all from Syria, and Mahyar Mahmud al-Uthmaan, 31, whose origin is unclear.

There were suggestions the video was an attempt to distract attention from recent military setbacks IS has faced, like losing the Iraqi town of Ramadi to state forces.

Lord Carlile, the former independent reviewer of terrorism legislation, told BBC News: "I think the video is propaganda and in many ways it demonstrates the weakness of Isil after the bashing they took in Iraq recently.

"Intelligence agencies will be seeking to identify both the adult and child."