London, Feb 27 (IANS): British Prime Minister David Cameron has defended the country's security services amid criticism they failed to stop Mohammed Emwazi, known as "Jihadi John", from joining Islamic State (IS) in Syria.

Cameron Friday said the security services made "incredibly difficult judgments" on Britain's behalf, BBC reported.

His comments came after it emerged Emwazi was known to authorities.

The prime minister said he would not comment on specific cases but urged the public to back the security services.

Emwazi, who is in his mid-20s, first appeared in a video last August, when he apparently killed US journalist James Foley.

He was later thought to have been pictured in the videos of the beheadings of British aid worker David Haines, US journalist Steven Sotloff, British taxi driver-turned-aid worker Alan Henning, and American aid worker Abdul-Rahman Kassig, also known as Peter.

On Thursday, the Washington Post reported the masked IS militant known as "Jihadi John," who has appeared in several execution videos, has been identified as Kuwait-born Briton Mohammed Emwazi, according to people familiar with the matter.