Twitter faces local computer server demand by Russia
Russia's internet regulator has said that Twitter must store local users' data in the country.
The policy marks a change in position because the watchdog Roskomnadzor had previously said the step would not be necessary.
It cited a change in the social network's terms and conditions as the cause and the organisation is also pursuing Facebook over the matter.
A spokesman for Twitter declined to comment.
Moscow passed a law last year that demands internet services which store Russian citizens' personal data must do so using Russia-based servers from September 2015.
Roskomnadzor has the authority to block and fine sites that do not comply.
Supporters of the move say it will help protect the public's data and strengthen national security, but critics view it as part of the state's tightening of control over the internet.
The watchdog had said in July that the law would not affect Twitter, because it believed the company did not store personal data - but that has now changed.
"They changed their user agreement some months ago. And if you read that, people must provide a set of metadata, which in our understanding as a whole counts as personal data and [makes it possible] to identify an individual," the regulator's chief Alexander Zharov was quoted as saying by the Financial Times.
He added that he had yet to hear a reply from Facebook, which had been previously told it told would need to operate local data centres.
Complying with the law could prove costly for both firms.
It is not clear, however, whether Roskomnadzor would carry through its maximum sanction if they refused.
When the body's deputy head Maxim Ksenzov raised the prospect of banning Twitter earlier this year, the Russian Prime Minister Dmitry Medvedev responded by telling officials to "switch on" their brains.