A “dramatic” rise in the number of websites and social media platforms hosting racist content is enabling neo-Nazi and other ultra-right groups to disseminate hate speech and incite violence, a United Nations (UN) expert has warned.

Tendayi Achiume, the organisation’s special rapporteur for racism, said there had been a 600 per cent increase in the number of white supremacists espousing their views on Twitter since 2012.

This, coupled with the proliferation of thousands of websites dedicated to far-right movements, contributed to a “growing climate of intolerance”, she added.

“At the core of neo-Nazism ideologies lies a hatred of Jews, as well as many other racial, ethnic, and religious groups. These ideologies also vilify lesbian, gay, bisexual, transgender and intersex people, and persons with disabilities, and in some cases women,” she said.

The role social media plays in furthering racial violence and the far right was put under the spotlight at the weekend after a gunman shot dead 11 worshippers at a synagogue in Pittsburgh.

Prime suspect Robert Bowers who has entered a plea of not guilty to federal charges, was nonetheless an active participant on the Gab social media network, which was created in 2016 to counter the perceived left-leaning bias of traditional social media platforms.

The company, which has been repeatedly criticised for allowing violent and far-right content on its platform, was taken offline after the shooting following a decision by the hosting provider, GoDaddy.

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But the likes of Twitter and Facebook have also faced criticism for not doing enough to counter hate speech on their portals.

Facebook announced earlier this week that it had banned several groups associated with the far-right political group known as the “Proud Boys”, including one with more than 20,000 members.

The tech company’s website said “violent or dehumanising speech, statements of inferiority, or calls for exclusion or segregation” were banned.

Twitter policy states the firm is “committed to combating abuse motivated by hatred, prejudice or intolerance, particularly abuse that seeks to silence the voices of those who have been historically marginalised”.

But Ms Achiume said tech companies still needed to do more to ensure their platforms did not provide a safe haven for extremist groups or allow the spread of “extremist, sensationalist, conspiratorial” news.

“Action is required by more states to implement anti-hate speech laws and ensure equality and non-discrimination including online, in accordance with international human rights law,” she said. “Criminal and civil penalties alone will not put an end to racial and xenophobic intolerance.