Russia's FSB state security service said on Monday it had detained an American citizen suspected of spying in Moscow and had opened a criminal case against him.

The FSB said the American had been detained on December 28 but it gave no details of the nature of his alleged espionage.

The English-language service of TASS news agency named the American as Paul Whelan but Reuters was unable to independently confirm the exact spelling of his name.

Russia's foreign ministry told TASS it could not provide further detail on the case, but said the U.S. Embassy in Moscow had been informed.

The U.S. Embassy would not comment directly, referring inquiries to the State Department in Washington.

Under Russian law, espionage can carry between 10 and 20 years in prison.

Earlier this month Russian national Maria Butina pleaded guilty in a U.S. court to a conspiracy charge in a deal with prosecutors, and admitted to working with a top Russian official to infiltrate American conservative activist groups and politicians as an agent for Moscow.

U.S. special counsel Robert Mueller in July indicted 12 Russian intelligence officers on charges related to hacking Democratic Party computer networks in 2016.

In February he charged 13 Russians and three Russian companies as part of a criminal and espionage conspiracy to tamper in the election to support Trump and disparage his Democratic opponent Hillary Clinton.

Russia has denied interfering in the election. Trump has denied colluding with Moscow.

Russia's relations with the United States plummeted when Moscow annexed the Crimean peninsula from Ukraine in 2014 and Washington and Western allies have imposed a broad range of sanctions on Russian officials, companies and banks.

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