MOSCOW — A Moscow court began hearing the espionage case against former U.S. Marine Paul Whelan on Monday. The trial opened about two weeks later than originally planned due to strict coronavirus lockdown measures in place in the Russian capital.

Whelan's defense team requested a further postponement of the trial Monday, until the lockdown aimed at stopping the spread of the disease is lifted, but the court ruled to reconvene in one week.

Olga Karlova, one of Whelan's defense attorneys, told CBS News that during the first hearing the prosecution formally presented its charges, and Whelan pleaded not guilty.

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"Paul said that he believes he was framed and that [the case] was a provocation against him," Karlova said, adding that her client appeared to be in good spirits.

Whelan, 50, was arrested at a central Moscow hotel in December 2018. Russian investigators claim he was caught red-handed after receiving a USB drive containing classified information. Whelan's family says he came to Russia to attend a friend's wedding and believed the USB drive handed to him by an acquaintance contained vacation photos.

Paul Whelan, a former U.S. Marine accused of spying and arrested in Russia stands inside a defendants' cage during a hearing at a court in Moscow on August 23, 2019. KIRILL KUDRYAVTSEV

The other defense attorney, Vladimir Zherebenkov, raised concerns over Whelan's health. He said the American, who suffers from a preexisting hernia, had asked the court to allow him a visit by the U.S. Embassy doctor, according to Russia's Interfax news agency.

The Monday hearing was held behind closed doors; the case has been declared classified by the Russian authorities. Press and U.S. Ambassador to Russia John Sullivan were barred from entering the court building due to the lockdown restrictions.

U.S. officials have repeatedly called for Whelan's immediate release, and the U.S. Embassy has been critical of Russia's treatment of the prisoner, who's being held in the notorious Lefortovo detention center and hasn't been permitted to call his family since his arrest.

The prosecution is expected to start presenting its evidence in the case and calling witnesses next week, the lawyers said.

Whelan, who also holds British, Irish and Canadian citizenship, faces up to 20 years in prison if convicted of espionage.

At the time of his arrest, Whelan was director of global security for Michigan-based automotive components supplier BorgWarner. He spent 14 years in the U.S. Marine Corps before being discharged in 2008 for bad conduct, according to the military. He served in Iraq for several months in 2004 and 2006.