The Islamic State has beheaded a third person from the West, British aid David Haines as it threatens another of Britain's children to be next in line amid their reign of terror.

A gruesome video was posted over the Internet displaying how an IS masked killer, known as Jihadi John decapitated 44-year-old Haines as the group continues to mock the Western giants following the death of two American journalists.

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Before he was killed, Haines, who was wearing the same orange outfit as the slain American journalists, was forced to read a script stating that British Prime Minister David Cameron is 'entirely responsible for my execution.'

"You entered voluntarily into a coalition with the United States against the Islamic State, just as your predecessor Tony Blair did, following a trend amongst our British prime ministers who can't find the courage to say no to the Americans," he said.

At the end of the two-and-a-half-minute footage, Jihadi John spoke out in his noticeably British accent and threatened to kill another British hostage identified as Alan Henning.

On Sunday, Cameron lauded the slain British aid for dedicating his life to helping those caught in the middle of fatal wars, dubbing him a hero.

He also expressed his disgust at the idea that Jihadi John, who was presumed to be British-born because of his East London twang, had the gull to carry out such acts of terrorism against his own people.

The video that shocked many in both Britain and other parts of the world was introduced by a clip showing Cameron condemning the Islamic State group.

Based on the three execution videos showing Haines, Americans James Foley and Steven Sotloff, the Islamic State group follows a specific pattern in the manner of execution, the scripts, as well as the executioner and victims' outfits.

Each also included a threat to kill another hostage who is shown briefly before the video ends.

TagsBritish, IS, threat, beheading