Russia's telecom regulator on Monday opened a civil case against Facebook and Twitter over their alleged failure to explain how they'll comply with data laws.
Roskomnadzor wants to know how and when the social media sites will comply with legislation requiring all servers that store Russians' personal data to be located in the country, it confirmed in an emailed statement.
Facebook and Twitter have a month to outline their plans, Roskomnadzor boss Alexander Zharov according to news agency Interfax.
Both companies sent formal responses to the watchdog's request for details, according to Roskomnadzor's email.
"They do not contain any specifics about the actual execution of the law at the moment, nor the timing of the execution of these rules in the future," it wrote. "In this regard, today Roskomnadzor starts against both companies' administrative proceedings."
If they don't comply, they'll reportedly be hit with small fine of 5,000 rubles (around $75, £60 or AU$105) each and given six months to a year to localize the data.
"Millions of people use Facebook in Russia to stay in touch with their family, friends and businesses. Facebook is in touch with relevant Russian governmental bodies regarding its activities in Russia," a Facebook spokesperson said in a statement.
Twitter declined to comment.
Currently, Russia issues small fines or blocks sites that fail to comply with its data laws, according to Reuters. In 2016, Roskomnadzor over its failure to store citizens' data on Russian servers, and the social network remains banned in the country after .
Russia may also start increasing the fines it imposes on tech companies, Reuters reported in November.
First published at 4:03 a.m. PT.
Updated at 8:56 a.m. PT: Adds Facebook statement.