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Isis's new video has featured Jihadi John leading two different beheadings for the first time.
As well as being shown standing over the severed head of American aid worker Abdul Rahman Kassig, the British Islamic State fighter also appears in a graphic mass beheading earlier in the video.
Jihadi John – the nickname of an unnamed British Islamic State militant – is shown alongside 18 other Islamic State militants, leading the systematic beheadings of up to 18 prisoners.
The video, released by al-Furqan Media Foundation, the Islamic State's media and propaganda organisation, states the victims are officers and pilots from the Syrian Arab Army of President Assad. The executed men are wearing matching blue uniforms and shown being led in a line, each prisoner being dragged by an Islamic State fighter.
None of the executioners, apart from Jihadi John, are masked and all appear to be wearing khaki desert fatigues. Each fighter drags their victim in a line before collecting a matching black hunting knife from a wooden box.
The victims are walked by their executioners in a line through the arid terrain and are forced to bend down on the ground.
Jihadi John is shown standing in the middle of the line, with his own victim kneeling before him. It is the first time he is seen beheading prisoners from the Assad army, rather than journalists and aid workers from America and Great Britain.
Jihadi John declares: "To Obama, the dog of war. Today we are slaughtering the soldiers of Bashar and tomorrow we will slaughter your soldiers.
"With Allah's permission, we will break this final and last crusade and the Islamic State will soon, as your puppet David Cameron said, begin to slaughter your people on your streets."
His voice appears to have been distorted deliberately in an attempt to hide his identity.
Slow-motion camera work is used to show close-ups of the terrified victims. Jihadi John gives a single word order in Arabic and all of the Islamic State fighters crouch down. They force their victims to the lie flat on the ground, pulling up their heads to expose their necks.
The camera focuses on Jihadi John, as he and all the other militants begin to cut off the heads of the Syrian Army soldiers in a synchronised mass beheading.
Jihadi John stops to peer up at the camera as his victim is shown dying.
The horrific scene is concluded with very graphic close-up shots of the beheading, before a final shot showing the bodies lying in a line with the victims' severed heads lying on top of the bodies.
The executioners are all shown holding clean knives and each is shown in a close-up shot. Their identities are not currently confirmed, but they appear to be nationals from around the world. One of the executioners in the video is believed to be a French fighter, known by the nom de guerre Abu Abdullah al-Firansi.
The beheading of the Syrian pilots is concluded with a voiceover statement from Islamic State's leader Abu Bakr al-Baghdadi, who boastfully says: "Know that we have armies in Iraq and an army in Sham (Syria) of hungry lions whose drink is blood and play is carnage."
The location of the video is currently unconfirmed, but it is alleged that the mass beheadings were carried out in Dabiq, a small town in the countryside in Syria's Aleppo province.
Dabiq is deeply symbolic to Islamic State as they have previously referred to it being the place for "the Final Hour" – the location where Armageddon will begin – according to the deeds of the Prophet, the Hadith.
A US official has confirmed that the video is authentic. The Kassig family have asked for privacy to mourn for the loss of their son, Abdul Rahman.