London, Nov 26 (IANS): Former director of British intelligence agency M16 has said it is unfair to expect Facebook (FB) to monitor messages for terrorist intent and report them to the security services.

Richard Barrett, a former counter-terrorism chief at MI5 and MI6, said he doubted the capacity of Facebook to sift through the volume of content handled by the website each day, as well as that of the security services to deal with the amount of information that would be referred to them if an obligation was placed on internet companies.

Barrett's comment came in the wake of British Prime Minister David Cameron's statement made in the light of a parliamentary report regarding the killing of soldier Lee Rigby, the Guardian reported.

The social media site, and other internet companies, are facing demands to police their content on behalf of the state after an official report into the death of Rigby said one of his killers wrote on a website - later named as Facebook - of his desire to slaughter a soldier, without the security services knowing.

The report published Tuesday by the parliamentary intelligence and security committee (ISC) said the authorities were never told that one of the killers, Michael Adebowale, wrote of his murderous intent six months before he and his accomplice, Michael Adebolajo, brutally attacked Rigby in May 2013 near his military barracks and tried to behead him.

Cameron said it was the companies' "social responsibility" to report when their networks were being used "to plot murder and mayhem" and vowed action.

Reacting in the wake of Cameron's statement, Barrett told BBC Radio 4's Today programme that Facebook has about one and a third billion users and about five billion posts a day so clearly on a worldwide basis "it would be almost impossible to deal with the amount of stuff that was referred".

"And even in the United Kingdom there are about 25 million users of Facebook and so let's say possibly about 125 million posts a day.

"And even if you take out all the pictures of kittens which were put up you'd still be left with an awful lot to go through and then quite a percentage of those perhaps would be passed on for the police or security services to look at. So it would be an enormous task, I think."