Russia's internet regulator has said Twitter must store local user data in the country.
The policy marks a change in position, as watchdog Roskomnadzor had previously said the step would not be necessary.
A change in the social network's terms and conditions was cited as the cause, and the organization is also seeking something similar from Facebook on the issue.
A Twitter spokesperson declined to comment.
Moscow passed a law last year requiring internet services that store personal data of Russian citizens to use Russia-based servers by September 2015.
Roskomnadzor has the authority to block and fine non-compliant sites.
Supporters of the move say it will help protect citizens' data and strengthen national security, but critics see it as part of the state's tightening control over the internet.
The watchdog had said in July that the law would not affect Twitter, because they believed the company did not store personal data – but that has now changed.
“They changed their user agreement a few months ago. And if you read it, people will have to provide a set of metadata, which in our understanding as a whole counts as personal data and [making it possible] to identify an individual,” said the head of the regulator Alexander Zharov, according to the Financial Times.
He added that he is still waiting for a response from Facebook, which had previously said it should operate local data centers.
Complying with the law could prove costly for both companies.
It is unclear, however, whether Roskomnadzor will face the maximum penalty if they refuse.
When the body's deputy head Maxim Ksenzov raised the prospect of banning Twitter earlier this year, Russian Prime Minister Dmitry Medvedev responded by telling officials to "turn on" their minds.
