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The sister of jihadi Abu Rumaysah says she will "kill him myself" if he is confirmed as the masked Briton in a new Islamic State video in which five hostages are executed.

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

She 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.

(Image: ITV)

She also pleaded with the UK authorities to provide an alternative for those who have been radicalised and travelled abroad, to help them return home.

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."

Intelligence agencies are desperately seeking to identify the new executioner after footage emerged last night of an Islamic State killer with an English accent threatening to attack the UK before executing four men accused of being British spies.

The video of the masked man ranting before shooting dead the hostages echoed clips of infamous ISIS figure Mohammed Emwazi - who became known as Jihadi John.

And today speculation continued as to the new executioner's identity.

Dhar was detained alongside radical hate preacher Anjem Choudhary on suspicion of supporting the banned extremist group al-Muhajiroun in 2014.

Police carried out raids on a number of addresses in London detaining 11 men.

Despite his arrest, it appears Rumaysah managed to flee the country and join jihadists.

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Raffaello Pantucci, director of international security studies at Royal United Services, said the executioner could be from London and well-educated.

He said: "He sounds a bit like Abu Rumaysah from Al-Muhajiroun videos.

"From watching him in Al-Muhajiroun videos and this new video he sounds very similar."

Dhar - a dad-of-four from Walthamstow, East London - later posted a picture of himself cradling his newborn baby son on Twitter while holding a machine gun in his other hand.

A caption read: “(God) blessed me with a healthy baby boy in the Islamic State.

"Another great addition to the Islamic State. He’s definitely not British.”

As many as 66 British people are believed to be in Syria or Iraq to join up with ISIS, according to the BBC.

Among those who have fled the country are Abu Rumaysah, one of a number of men arrested in September 2014 in an anti-terror operation and released on bail.

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Other men who have left the UK in recent years include brothers Nasser and Aseel Muthana, as well as their friend Reyaad Khan, all from Cardiff.

Former medical student Nasser appeared in a propaganda video for Islamic State in June 2014 with Khan.

(Image: Media Wales)

The pair, both 20 when they fled for Syria, were later joined by Nasser's teenage brother, Aseel.

The brothers left behind their stunned family in the Welsh capital.

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And a family of 12 from Luton went missing in July last year missing amid fears they travelled to Syria to join Islamic State.

The group flew to Bangladesh for a holiday in May but failed to return, and an ISIS representative issued a statement claiming to be on behalf of the family.

The latest ISIS video, which includes footage too graphic to be shown, has been condemned in Britain.

Labour MP Sadiq Khan said: "The evil and disgusting ISIS propaganda video shows why we need to do much more to tackle radicalisation in Britain."

A Foreign Office spokesman said: "We are aware of the video and are studying its contents."