Whelan, who has denied the charges, could face up to 20 years in prison if found guilty. His lawyer said the 48-year-old was "behaving in a constructive manner" and that investigators were treating him in a "humane and professional way". The charges came a day after the US ambassador to Russia Jon Huntsman met Whelan at the Lefortovo prison in Moscow.
"Ambassador Huntsman expressed his support for Mr Whelan and offered the embassy's assistance," a State Department spokesperson said following the visit. The United States has been cautious in its public comments on the case, with Secretary of State Mike Pompeo saying earlier Wednesday that Washington was trying to learn more about the situation.
Born in Canada, Whelan was arrested last Friday "while carrying out an act of espionage", according to Russia's FSB domestic security service. His family has denied he is a spy, saying he was visiting Moscow to attend the wedding of a fellow ex-Marine with a Russian woman. Whelan's employer, US-based automotive components supplier BorgWarner, said that he is the firm's director of global security.
The arrest came after President Vladimir Putin accused Western nations of using espionage cases to try to undermine an increasingly powerful Russia. US intelligence services have accused Moscow of interfering in the 2016 presidential election.