The FSB did not name the person or provide any other details, nor did it say whether the suspect had been arrested.

“The Federal Security Service of the Russian Federation has opened a criminal case against a US citizen on the grounds of a crime under Article 276 of the Criminal Code of the Russian Federation – ‘Espionage’,” the FSB said in a statement.

“The American is suspected of collecting intelligence on biological topics directed against the security of the Russian Federation,” the FSB said.

It did not elaborate on the nature of the allegations.

The US embassy could not be immediately reached for comment.

A number of US citizens have been detained on criminal charges in Russia in recent years, with some ultimately freed in exchange for Russians detained in the US. In December, US basketball star Brittney Griner was freed after being sentenced to 9 years in prison on drug charges.

She was exchanged for Viktor Bout, a Russian arms dealer who spent 14 years in US jail for arms trafficking, money laundering and conspiring to kill Americans.

However, espionage cases are seen as especially fraught, with Russian security services often unwilling to release those it accuses of being spies.

Russia has so far refused to swap US Marine Corps veteran Paul Whelan, who was arrested by the FSB in 2018 on espionage charges and sentenced to 16 years in prison in 2020.