The black-clad militant brandishing a knife and speaking with an English accent was shown in videos released by Islamic State (IS) apparently decapitating hostages including Americans, Britons and Syrians.
               
"When there are people anywhere in the world who commit appalling and heinous crimes against British citizens, we will do everything we can with the police, with the security services, with all that we have at our disposal to find these people and put them out of action," Cameron said.
               
Cameron refused to go into the unmasking of "Jihadi John" as 26-year-old British militant Mohammed Emwazi, but said that people should get behind the security services, which he praised as impressive and dedicated to defending Britain.
               
Emwazi was known to the security services, which had tried to recruit him, according to prisoners' group Cage. The case has sparked debate about whether the security services let him slip through their grasp to join IS in Syria.
               
Dressed entirely in black, a balaclava covering all but his eyes and the bridge of his nose and a holster under his left arm, Jihadi John became a menacing symbol of Islamic State brutality and one of the world's most wanted men.
               
He used videos to threaten the West, admonish its Arab allies and taunt President Barack Obama and Cameron before petrified hostages cowering in orange jump suits.

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