Updated 3.44pm: "Jihadi John", the masked Islamic State militant apparently responsible for the beheading of western hostages, has been named as London man Mohammed Emwazi by the Washington Post and the BBC.

Britain's Metropolitan Police would not confirm Thursday's reports, which identified the suspect as Kuwaiti-born Emwazi, who grew up in west London.

"We are not going to confirm the identity of anyone at this stage or give an update on the progress of this live counter-terrorism investigation," said Richard Walton from the Met's Counter Terrorism Command.

Contacted by AFP, the interior ministry could not immediately comment on the reports.

The Guardian and the BBC both reported the identity, without citing their sources.

British media had previously suggested "Jihadi John" could be a different British jihadist.

Emwazi, in his mid-20s, was identified to the Washington Post by friends and others familiar with the case, with one close acquaintance telling the paper: "I have no doubt that Mohammed is Jihadi John".

The suspect is from a middle class family and earned a degree in computer programming before travelling to Syria around 2012, according to the report.

He apparently became radicalised after being detained by authorities following a flight to Tanzania and being accused by British intelligence officers of trying to make his way to Somalia.

"Jihadi John", named after Beatle John Lennon due to his British accent, is believed to be responsible for the murders of US journalists James Foley and Steven Sotloff, British aid workers David Haines and Allan Henning and American aid worker Abdul-Rahman Kassig.

He also appeared in a video with the Japanese hostages Haruna Yukawa and Kenji Goto, shortly before they were killed.

In the videos posted online, he appears dressed all in black with only his eyes exposed, and wields a knife while launching tirades against the West.

Following the appearance of the report online, UK-based terror expert Shiraz Maher tweeted "Well, that's it. The name is out. Jihadi John has been identified as Mohammed Emwazi."

British intelligence officers estimate that there are around 500 homegrown militants fighting for IS in Syria and Iraq.

The family of Steven Sotloff say they are relishing seeing his killer face justice.

The family has issued a statement after his Islamic State killer Jihadi John, was unmasked as London computer programmer Mohammed Emwazi.

The Sotloffs say they have full faith that the American intelligence community and law enforcement agencies will apprehend him.

Emwazi is believed to have beheaded five Islamic State prisoners, including Stephen Sotloff.