The lawyer of a former US marine accused of spying by Russia has said his client had been misled before his arrest and believed that a USB drive handed to him in a hotel room contained holiday photos rather than secret information.

Russia's Federal Security Service arrested Paul Whelan, who holds American, British, Canadian and Irish passports, from a Moscow hotel room on December 28.

Whelan appeared in a Moscow court on Tuesday, where a judge rejected his plea for release on bail. If found guilty of espionage, he could be jailed for up to 20 years.

Whelan, who denies the charges, was taken into custody after receiving a USB drive allegedly containing a list of all the employees of a secret Russian state agency, Russian online news portal Rosbalt.ru reported earlier this month.

The website cited an unnamed Russian intelligence source as saying that Whelan had been spying for 10 years, using the internet to identify targets he could obtain information from, and that the list he was caught with had long been of interest to US spies.

Russian Foreign Minister Sergei Lavrov appeared to support that version of events, telling reporters Whelan had been "caught red-handed" carrying out "specific illegal actions" in his hotel room.

His lawyer Vladimir Zherebenkov said Whelan did not know the contents of the USB drive.

"On a USB drive, Paul received information that ... I confirm is a state secret," Zherebenkov told journalists.

"In reality, Paul was supposed to receive from the person information that is not a state secret," he said, indicating he was expecting to be given cultural material and photographs.

The lawyer said Whelan was arrested before he had a chance to see what was on the drive.

There are reports that Whelan may have been arrested in retaliation for the arrest of Russian woman Maria Butina, a gun rights campaigner who was convicted in December of being a "foreign agent" and likely faces six months in jail, by the US.

Moscow has rejected the idea Whelan could be exchanged for any US prisoner, saying it does not treat people as "pawns" in diplomatic games.

Zherebenkov said he expected any trial to last for at least six months and that there could be no talk of any exchange until it was completed.