The Islamic State will slaughter your people in your streets': New 'Jihadi John' threat to Britain and the US after killing US Aid worker

             A masked jihadist today issued a chilling threat to the West in a video purporting to show the severed head of a U.S. aid worker and a mass murder of Syrian soldiers.

Speaking in the now-familiar London accent of Islamic State executioner-cum-spokesman Jihadi John, the militant issues the bloodcurdling threat in the group's latest propaganda video.

Later in the same video, which emerged today, it is claimed that another Western hostage, the aid worker and former U.S. army ranger Peter Kassig, has also been executed. A severed head said to Mr Kassig's is shown at the end of the near 16-minute long clip, which was uploaded to video sharing sites and publicised via Twitter.

           But it is at the beginning of an earlier scene, showing the graphic mass murder of Syrian air force personnel, that the masked militant issues his threat against civilians in the U.S. and UK. It shows the men led to a place in the desert by bearded, uniformed jihadis who each pick up a knife which they will use to saw the heads off their victims.

As militants stand in a row with their hostages kneeling in front of them, Jihadi John says: 'To Obama, the dog of Rome. Today we are slaughtering the soldiers of Bashar and tomorrow we will be slaughtering your soldiers.

'And with Allah's permission we will break this final and last crusade. And the Islamic State will soon, like your puppet David Cameron said, begin to slaughter your people in your streets.' Prime Minister David Cameron said he was 'horrified by the cold blooded murder' of US hostage Peter Kassig, and claimed that Islamic State militants 'have again shown their depravity'.  

           Mr Cameron said: 'These beheadings show once again what a depraved organisation this is.

'Like others who have been murdered in cold blood by ISIL, Abdul-Rahman Kassig was a selfless humanitarian worker who had gone to the region to help care for those fleeing the Syrian conflict.

'This underlines the deplorable depths to which these terrorists are prepared to go - savagely murdering a compassionate man.'

          The Prime Minister said his 'heart goes out' to Mr Kassig's family. 

The White House says U.S. intelligence agencies are trying to determine the authenticity whether the video is real.

National Security Council spokeswoman Bernadette Meehan says that if the video is authentic, the White House would be 'appalled by the brutal murder of an innocent American.'

She says the White House expresses its deepest condolences to Mr Kassig's family and friends

Mr Kassig's family, in the U.S. state of Indiana, said they were awaiting confirmation of the reports about their 'treasured son'.

          His parents, who have campaigned tirelessly for his release, said in a statement: 'We are aware of the news reports being circulated about our treasured son and are waiting for confirmation from the government as to the authenticity of these reports. We will have no other statement at this time and ask that you please respect our privacy.

          'The family respectfully asks that the news media avoid playing into the hostage takers’ hands and refrain from publishing or broadcasting photographs or video distributed by the hostage takers.

'We prefer our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause.'

      Via.daily mail