British Prime Minister David Cameron has defended security services and vowed to defeat Islamic extremists after media reports named Islamic State executioner "Jihadi John" as London graduate Mohammed Emwazi.

"We will do everything we can with the police, the security services, with all that we have at our disposal, to find these people and put them out of action," Cameron said.

"All of the time (the security services) are having to make incredibly difficult judgments and I think basically they make very good judgments on our behalf." As the Islamic State group militant believed to be responsible for beheading at least five Westerners was finally identified by media and experts, families of the hostages said they hoped it would lead to him being brought to justice.

"My only hope is that the revelation of his identity will lead to his arrest," Dragana, the widow of David Haines, told AFP from her home in Croatia. Haines' daughter Bethany, however, said victims' families would feel closure only "once there's a bullet between his eyes".

Britain's domestic spy agency MI5 came under scrutiny following the revelations about Emwazi, a Kuwaiti-born computing graduate who had lived in London since the age of six and had reportedly been tracked since at least 2009.

London mayor Boris Johnson, meanwhile, accused civil rights group Cage of "an apology for terror" for blaming Emwazi's radicalisation on his alleged detention and "harassment" by the British security services.

Cage, which published years of correspondence with Emwazi, said he had become radicalised following a post-graduation trip to Tanzania in 2009 when he was accused of seeking to join militants in Somalia.

It also alleged that MI5 had launched a failed bid to recruit him.

"It was incredible that people could stand up and pretend that somehow it was the fault of the security forces," Johnson said.

An acquaintance of Emwazi who worshipped at a mosque near his London home described him as a "strict" Muslim who prayed up to five times a day.

In the gruesome Islamic State videos posted online, the masked executioner appears dressed all in black with only his eyes exposed, brandishing a knife while launching tirades against the West.

He is believed to be responsible for the murders of US journalists James Foley and Steven Sotloff, British aid workers David Haines and Alan Henning and American aid worker Abdul-Rahman Kassig.

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