Russia’s communications watchdog has threatened to fine Facebook, Google and Twitter and block their services under a controversial law on blogging.

In a letter to executives on on Monday, the director of the communications oversight agency warned that the three US companies could face sanctions if they continued alleged illegal activities in Russia, Izvestia newspaper reported on Wednesday.

Any action could affect a number of social media sites: besides its eponymous social network, Facebook also owns the photo-sharing service Instagram, while Google owns YouTube, BlogSpot and Google+. Facebook and Twitter, in particular, have been instrumental to organisers of opposition protests in Russia, where the major television news channels are controlled by the state.

A spokesman told the state news agency RIA Novosti that the watchdog’s complaints related mainly to deleting pages with extremist materials and receiving information under what is known as the “bloggers law”. This 2014 legislation requires popular bloggers to register their real identities with the authorities, a measure that prominent bloggers say is designed to have a curb free speech and criticism of the regime.

The agency’s deputy director, Maksim Ksenzov, had issued a warning to the three companies on 6 May, telling them they were in violation of the bloggers law because they had not provided requested data on the number of daily visitors to several users’ pages, as well as information allowing the authorities to identify the owners of accounts with more than 3,000 daily visitors.

According to the law, the agency can fine a violating organisation up to 300,000 roubles (£3,850); a second infringement can incur a fine of up to 500,000 roubles or a suspension of its operations for up to 30 days.

If the companies did not take steps to delete from their sites “information containing calls to participate in mass rioting, extremist activities” or unsanctioned public events, the watchdog would “limit access to the information resource where that information is posted”, Ksenzov warned.

Although Russia’s four major internet providers are reportedly able to block the URL of specific pages on social networks and other websites, regional providers with less exact technologies have in the past been forced to block entire services due to one offending page. According to the communications oversight agency’s data, 70% of registered internet providers are able to block separate URLs, Izvestia reported.

Since the start of President Vladimir Putin’s third term in 2012, the government has launched a crackdown on the internet in Russia, passing laws that give state supervisory bodies wide-ranging powers to to regulate and block websites.

Several news sites critical of the Kremlin have been blocked in Russia, including Grani.ru, EJ.ru and Kasparov.ru, which was founded by self-exiled chess grandmaster and activist Garry Kasparov. Facebook blocked 55 pages at the communication oversight agency’s request in the second half of 2014, including one inviting people to a rally in support of opposition leader Alexei Navalny.

Google was forced to move some servers to Russia this year under a law requiring Russians’ personal data to be stored on its territory.