A man named only as Saleh told Sky News the masked militant who appears in several beheading videos was a senior figure in the extremist organisation responsible for murdering foreign captives.

Jihadi John, who was recently unmasked as 26-year-old Londoner Mohammed Emwazi, was last seen in a video posted online which appeared to show Japanese hostage Kenji Goto lying dead.

Speaking to the broadcaster in broken English from Turkey, after fleeing IS, Saleh said: "When he killed Kenji Goto I live showed this (saw this) but not near, from a little (distance).

"The big boss was there with them. Turkish man say 'put this camera there, change place there' but John (was) the big boss. All time, all time say to all 'fastly, fastly, fastly, we should finish'. So respect him. Only he talks orders - others do."

Appearing with his face covered, the man added Jihadi John won respect for murdering foreign prisoners.

He said: "Maybe because he use the knife. I cannot understand why he is so strong. One man can kill and all people will respect. A Syrian man anyone [in IS] can kill. But strangers [foreigners], only John."

And hostages were routinely subjected to mock executions, with their captors telling them they would not be killed as it was a show for the camera, according to Saleh.

Kuwait-born Emwazi had been pinpointed as a potential terrorist by the British authorities but was nonetheless able to travel to Syria in 2013 and join a group responsible for the murder of several Western hostages.

In email exchanges with a journalist, the University of Westminster computer programming graduate said he considered suicide after coming face to face with what he suspected to be a British spy as he attempted to sell a laptop computer in 2010.

Jihadi John rose to notoriety after he first appeared in a video posted online last August, in which he appeared to kill American journalist James Foley.

Dressed in black with a balaclava covering all but his eyes and the ridge of his nose, and a holster under his left arm, he reappeared in videos of the beheadings of US journalist Steven Sotloff, British aid workers David Haines and Alan Henning, and American aid worker Peter Kassig before featuring in a video with Japanese hostages Haruna Yukawa and Kenji Goto, shortly before they were killed.