Russia's FSB state security agency says it has arrested a US citizen "caught spying" in Moscow.
It named him as Paul Whelan, saying he was arrested in Moscow on 28 December and charged with "espionage".
The FSB gave no further details.
The US state department says it has requested consular access for the man held in Moscow, Reuters news agency reports. The US has not confirmed his name, but says it has been officially notified about the arrest.
If found guilty, he faces between 10 and 20 years in jail, Russia's Tass news agency reports.
Spying accusations have been a persistent feature of Russia's relations with the US and UK this year.
Associates of President Donald Trump are under investigation in connection with Robert Mueller's special counsel inquiry into possible Russian ties to the Trump presidential campaign.
Earlier this month a Russian gun rights activist held in the US, Maria Butina, pleaded guilty to conspiracy. US prosecutors say she acted as a Russian state agent, infiltrating conservative political groups.
In March the UK and its Western allies expelled more than 100 Russian diplomats, in response to the poisoning of former Russian spy Sergei Skripal and his daughter Yulia in Salisbury. The UK government accused the Russian state of carrying out the nerve agent attack.
Russia denied this and retaliated by expelling dozens of Western diplomats.