A US journalist has been barred from entry to Russia, in what his employers say is a "fundamental violation of the right of free speech".
David Satter tried to get his Russian visa renewed while on a trip abroad, but said he was told the authorities in Moscow found him "undesirable".
However, Russia said he had previously violated visa regulations.
The BBC's Steve Rosenberg in Moscow says it is not clear whether he was barred for political or other reasons.
There is sensitivity in Russia ahead of the Winter Olympics, which begin in the city of Sochi in a few weeks.
However, Russian President Vladimir Putin has made a number of concessions ahead of the Games, including freeing some political opponents from prison and assuring gay athletes of their safety despite a tightening of the laws around homosexuality.
David Satter was previously a Financial Times correspondent in Moscow, now working as an adviser to the US government-funded Radio Free Europe/Radio Liberty.
Our correspondent says Satter has long been a critic of the Kremlin.
RFE/RL President Kevin Klose complained about a violation of free speech and said the US embassy in Moscow had lodged a diplomatic protest.
Satter, who has authored several books about Russia, including one whose title referred to the "Rise of the Russian Criminal State", said: "They know me very, very well."
"I have been writing about Russia, writing about the Soviet Union almost for four decades. To say that I'm not allowed on the territory of the Russian Federation at the request of the security services - this I haven't seen applied to a journalist in my entire career of writing about Russia," he told the Associated Press.
A source in the Russian foreign ministry told the BBC the decision not to allow Satter back into the country had nothing to do with his criticism of the authorities - and was linked to a violation of visa regulations by the journalist.
It said that on the expiry of a previous visa, he had failed to exit Russia on time.
In November, a Moscow court had fined him about £100 ($165) for being in Russia without a valid visa, and had ordered him to leave the country.
Satter has dismissed that explanation as "bureaucratic trickery".
He said he was assured his visa would be extended.