MOSCOW -- The Russian Foreign Ministry on Saturday criticized the demand by the US Embassy in Russia to release the suspected spy Paul Whelan out of health concerns.

"Letting a person go home, where they can receive the necessary medical help: does it only concern Paul Whelan? There are millions of prisoners in the US, including over 50 Russian citizens, many of them with health issues. Let them go," the Russian ministry responded to the US demand on Twitter.

Earlier in the day, the US Embassy posted a tweet urging the Russian side to release Whelan for him to receive proper medical help, after he complained about his health conditions at a hearing on Friday when Russia's Lefortovo district court ruled to extend his detention for another two months.

Whelan was detained in Moscow by the Russian Federal Security Service (FSB) on suspicion of espionage on Dec 28, 2018, although his family said he traveled to Moscow only to attend a friend's wedding.

A criminal investigation was initiated against him. If convicted, Whelan faces up to 20 years in jail.

The 48-year-old suspect was a US Marine and is currently the head of the security service of a firm producing automobile parts. He also has British, Irish and Canadian citizenship.