A US citizen has been detained in Moscow on suspicion of spying, Russia's FSB security service has said.
The FSB said the American had been detained "while carrying out an act of espionage" on 28 December.
A criminal case had been opened under article 276 of the Russian Criminal Code, which allows for a potential 20 years in prison.
An FSB statement identified the person in Russian, with the AFP news agency saying the name appeared to translate as Paul Whelan.
No other details were immediately available and the US embassy in Moscow has so far not commented.
The arrest comes amid a low point in relations between many Western countries and Russia following several spying controversies.
US authorities have accused Russia of interfering in the 2016 election and earlier this month convicted Maria Butina of acting as an illegal foreign agent.
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Prosecutors said she sought to develop ties with the Republican Party with the aim of influencing US foreign policy.
Russian agents were also accused of being behind the poisoning of former Russian double agent Sergei Skripal and his daughter in Salisbury earlier this year. Russia denied the claims.