Moscow (AFP) - Russia's media watchdog Roskomnadzor launched "administrative proceedings" Monday against US social media giants Facebook and Twitter, accusing them of not complying with Russian law, news agencies reported.

"Today, Roskomnadzor begins administrative proceedings against both companies (Facebook and Twitter)," the watchdog's head Alexander Zharov told the Interfax news agency.

The state regulator has repeatedly warned the companies they could be banned if they do not comply with a 2014 law requiring social networking sites to store the personal data of Russian users inside the country.

Zharov said Facebook and Twitter provided "no concrete information on localising the data of Russian users on the territory of the Russian Federation."

He added that the companies also did not provide a "timeframe" for when they plan to store the data of Russian users in Russia.

The 2014 law has caused widespread concern as it is seen as putting the information of Russian users at risk of being accessed by the country's intelligence services.

Russia has put increasing pressure on popular social networks, websites and apps in what opposition figures see as an attempt to silence the main forum for political debate and organising protests.

In April last year, thousands rallied in Moscow in support of internet freedom after Russian authorities tried to block access to the popular messaging app Telegram.

Telegram had refused to give state security services access to private conversations that are normally encrypted.

In 2016, Russian internet providers blocked the LinkedIn professional networking site after Roskomnadzor said it broke the data storage law.