Russia's communication watchdog, Roskomnadzor, opened "administrative proceedings" Monday against Facebook and Twitter for non-compliance with country’s data laws, Interfax news agency reported.
Roskomnadzor head Alexander Zharov is quoted as saying that U.S. social media giants have a month to comply or face legal proceedings.
According to Roskomnadzor, Facebook and Twitter have not explained how and when they would comply with legislation that requires all servers used to store Russians' personal data to be located in Russia.
Russia has introduced stricter internet laws in the past five years, among other things requiring search engines to share encryption keys with Russian security services.
In April last year, thousands rallied in Moscow in support of internet freedom after Russian authorities attempted to block access to the popular messaging app Telegram.
Telegram had refused to give state intelligence services access to private conversations which are usually encrypted.
