Former U.S. Marine Paul Whelan, who was detained by Russia's FSB security service on suspicion of spying, attends a court hearing in Moscow, Jan. 22, 2019.
Former U.S. Marine Paul Whelan, who was detained by Russia's FSB security service on suspicion of spying, attends a court hearing in Moscow, Jan. 22, 2019.

A court in Russia has denied bail to an American man being held on spying charges.

Paul Whelan, a former U.S. Marine, appeared in a Moscow court Tuesday to appeal his detention, after being arrested on suspicion of espionage in December.

The judge ruled Whelan must stay behind bars at least until the end of February. If convicted of spying, he could be sentenced to up to 20 years in prison.

Whelan's twin brother, David, released a statement for the family after the court proceedings were over.

"While we still lack any details from the Russian government about why Paul is thought to be a spy, and who provided him with the alleged state secrets, we are certain that he was entrapped and is not guilty of espionage. ... Unfortunately, today's ruling merely confirms that Paul will remain wrongfully detained for many more months."

The statement said Whelan is worried about his health and his ability to advocate for himself in English while detained.

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Paul Whelan looks through a cage's glass as he speaks to his lawyers in a court room in Moscow, Jan. 22, 2019.

A spokesperson at the U.S. State Department told VOA Tuesday that diplomatic officials are "closely following" Whelan's case, and U.S. embassy representatives attended Whelan's court appearance on Tuesday.

The spokesperson said the United States continues to urge Russia to follow international law and provide for a swift, fair and transparent judicial process for all detained U.S. citizens in Russia.

Whelan, whose lawyer said he is a frequent visitor to Russia, was detained at a hotel in Moscow on Dec. 28. He was accused of carrying Russian state secrets on a flash drive given to him by an unnamed person.

Whelan's Russian lawyer, Vladimir Zherebenkov, told reporters that Whelan was expecting the flash drive to contain travel information, but it instead contained information sensitive to the Russian government.

Whelan holds passports from the United States, Canada, Ireland and Britain. It is not clear for what country Russia accuses him of spying.

His family has said he is innocent and was in Moscow to attend a wedding.

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FILE - Maria Butina speaks at 2015 FreedomFest conference in Las Vegas, Nevada, July 11, 2015 in this still image taken from a social media video obtained July 19, 2018.

There is speculation the Russians might try to use Whelan as leverage for the release of Maria Butina, who pleaded guilty to infiltrating America's conservative political movement as a Kremlin-directed agent. Butina was convicted of acting as a foreign agent in the United States.

The Kremlin has dismissed the idea of Whelan being held for a possible prisoner swap.

Whelan, who is in his late 40s, was discharged from the U.S. Marines for bad conduct. He is a global security director for a U.S. auto parts company.