Russia's Sputnik news site says its website has been blocked in Turkey.
Users trying to access the site are reportedly receiving a message saying it is inaccessible pending "technical checks and legal assessments".
The agency's Turkish staff said no warning had been received from telecoms authorities.
Relations between Moscow and Ankara have worsened since Turkey shot down a Russian warplane near the Syrian border five months ago.
The news comes hours after a series of anti-Turkish statements from Russian President Vladimir Putin in the course of his annual televised phone-in.
Sputnik was launched by Russian state media in 2014 to target a global audience "who want a different perspective".
"We've sent a letter to the regulatory agency asking for the reasons," editor-in-chief Mahir Boztepe told Reuters news agency. "We were not expecting a ban at all."
Controversial legislation introduced in 2014 gave Turkish authorities the power to block websites for privacy violations without a court decision.
President Recep Tayyip Erdogan has been accused of presiding over an increasingly authoritarian administration.
Last month Turkey's biggest newspaper, Zaman, was taken over by the authorities after a court ruling placed it under state control. No explanation was given for the ruling.
Freedom of the press in Turkey
- Turkey ranks 149th amongst the 180 countries in the Reporters Without Borders' World Press Freedom Index 2015
- Media organisations in Turkey say that more than 30 journalists are currently behind bars
- Most are of Kurdish origin
- The government argues journalism in Turkey is among the most free in the world
- 28 October 2015
- 16 November 2014