The video triggered anti-American protests in North Africa and the killing of the US ambassador to Libya in an assault by Islamist gunmen on the Benghazi consulate and on a safe house on Tuesday night.

Ironically, Benghazi was at the centre of the uprising - supported by the Western powers, including the US - that toppled the despot Muammar Gaddafi last year.

Announcing Afghanistan's decision to ban YouTube, the government's communications chief, Aimal Marjan, said the site would remain shut to the Afghan public until the video was taken down.

The film, "Innocence of Muslims", was promoted by the controversial right-wing American pastor Terry Jones - notorious for threatening to burn the Koran in 2010.

Jones's threats triggered riots in Afghanistan that resulted in several deaths, as did the accidental burning of Korans at a US base in February.

Afghanistan's temporary banning of YouTube might not be welcomed by the advocates of unfettered freedom of speech but it is entirely explicable from a security point of view.

The sensitivities of Muslims worldwide to depictions of Mohammed are well known and the administrators of a war-torn country simply cannot take the risk of more people being killed because of a work of ''art'' that, by all accounts, amounts to hate speech.

This leads to an infinitely more perplexing question: at what point should the owners of social media take responsibility for the content that they allow to be put up on their sites when it amounts to hate speech or incites violence?

Surely the answer cannot be that they will wait for governments to act.