SCOTLAND Yard has warned people that downloading, viewing or passing on extremist material could be a criminal offence as YouTube took action to remove any video of James Foley's murder from its website.

Twitter, meantime, encouraged its users not to share videos of the brutal killing by the Islamic State (IS) and, using the hashtag #ISISmediablackout, it urged people instead to share photos of the smiling American photojournalist before his capture in Syria in 2012.

Dick Costolo, the social media site's chief executive, said it was taking action against accounts, which had spread the video, noting: "We have been and are actively suspending accounts as we discover them related to this graphic imagery."

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Thousands of people, including celebrities and scores of fellow journalists, took to social media to urge people not to give the terrorist group the oxygen of publicity.

Paul Conroy, a photographer for the Sunday Times, who was injured in Syria in 2012 by a shell that killed reporter Marie Colvin, said people spreading the video were doing IS's work for it.

"In many ways, the passing around of these pictures and the videos of James is doing what these people - these murderers - want you to do," he warned.

Actress Mia Farrow was among those who urged their Twitter followers to shun the footage, which showed the death of the 40-year-old.

She tweeted: "Blackout on group that murdered James Foley. Don't share video. Give them nothing. #RespectJames Foley,"

Earlier, the Metropolitan Police said: "Viewing, downloading or disseminating extremist material within the UK may constitute an offence under terrorism legislation."