By Tom Balmforth and Maria Kiselyova

MOSCOW (Reuters) – Russia said on Wednesday it was slowing down the speed of Twitter in retaliation for what it described as a failure to remove banned content, threatening to block the U.S. platform outright if it did not comply with its deletion demands.

The move escalates a growing stand-off between Moscow and U.S. social media firms. It comes weeks after Russian authorities accused Twitter and others of failing to delete posts that Moscow said illegally urged children to take part in anti-Kremlin protests.

In Wednesday’s announcement, Russia’s Roskomnadzor communications regulator referred to what it said was illegal content on Twitter containing child pornography, information about drug abuse and calls for minors to commit suicide.

Twitter said it was worried about the impact on free speech of the Russian action, and denied that it allowed its platform to be used to promote illegal behaviour.

“We remain committed to advocating for the Open Internet around the world and are deeply concerned by increased attempts to block and throttle online public conversation,” a Twitter spokesperson said in an emailed statement to Reuters.

“Let us be clear – we have a zero-tolerance policy regarding child sexual exploitation, it is against the Twitter Rules to promote, glorify or encourage suicide and self harm, and we do not allow the use of Twitter for any unlawful behaviour or to further illegal activities, including the buying and selling of drugs.”

Russia has traditionally taken a more hands-off role in policing the internet than neighbouring China. But as domestic political tensions have risen this year over the arrest and jailing of Kremlin critic Alexei Navalny – which triggered nationwide protests – it has signalled a tougher line.

Roskomnadzor said that as of Wednesday there were more than 3,000 posts containing illegal content on Twitter, which it accused of ignoring its deletion requests for years.

“The slowing down will be applied to 100% of mobile devices and on 50% of non-mobile devices,” the regulator said.

“If (Twitter) continues to ignore the requirements of the law, the enforcement measures will be continued… (right up to blocking it),” the regulator said.

Twitter was already under pressure in Russia after it was named as one of five social media platforms being sued for allegedly failing to delete posts urging children to take part in illegal protests, the Interfax news agency cited a Moscow court as saying on Tuesday.

‘CONTROL THE INFORMATION SPACE’

The Kremlin said there was no desire to block content but that companies had to abide by the law.

Some activists, however, said they believed the curbs were linked to recent protests.

“Of course the main motive is the increase in street protest action,” said Sarkis Darbinyan, an internet freedom advocate with the Roskomsvoboda group.

“It’s 10 years since the Arab spring this year… they’ve understood the internet is a driving force. Any desire to control the Russian internet is connected to the desire to control the information space.”

Navalny’s allies say they plan new protests in coming months.

Some government websites were unavailable for some Russian internet users shortly after the announcement about Twitter.

Telecoms operator Rostelecom said the disruption to several government websites, including those of the Kremlin and parliament, was not caused by new restrictions imposed on Twitter but by an equipment malfunction.

Vadim Subbotin, an official at Roskomnadzor, said it was possible that the authorities could target and slow down other internet platforms if they failed to comply with the law, the Interfax news agency reported.

Moscow has gradually introduced tougher internet laws in recent years, requiring search engines to delete some search results, messaging services to share encryption keys with security services and platforms to store user data on servers in Russia.