Russia's media watchdog Roskomnadzor has written to Google, Twitter and Facebook warning them against violating Russian Internet laws and a spokesman said yesterday that they risk being blocked if they do not comply with the rules.
Roskomnadzor said it had sent letters this week to the three US-based Internet firms asking them to comply with Internet laws which critics of President Vladimir Putin have decried as censorship.
"In our letters we regularly remind (companies) of the consequences of violating the legislation," said a spokesman for the organisation.
He added that, because of the encryption technology used by the three firms, Russia had no way of blocking specific websites and so could only bring down particular content it deemed in violation of law by blocking access to their whole services.
To comply with the law, the three firms must hand over data on Russian bloggers with more than 3,000 readers per day, and take down websites that Roskomnadzor sees as containing calls for "unsanctioned protests and unrest", the spokesman said.
Putin, a former KGB spy, once described the Internet as a project of the CIA, highlighting deep distrust between Moscow and Washington, whose ties are now badly strained.
He promised late last year not to put the Internet under full government control, but Kremlin critics see the Internet laws as part of a crackdown on freedom of speech since Putin returned to the Kremlin for a third term in 2012.
A law passed last year gives Russian prosecutors the right to block without a court decision websites with information about protests that have not been sanctioned by authorities.
Under other legislation, bloggers with large followings must go through an official registration procedure and have their identities confirmed by a government agency.