Russia has opened a criminal case against a United States citizen on suspicion of espionage, its Federal Security Service (FSB) says.
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 on Thursday.
"The American is suspected of collecting intelligence on biological topics directed against the security of the Russian Federation."
It did not elaborate on the nature of the allegations.
The US embassy could not immediately be reached for comment.
The US State Department said it was aware of the "unconfirmed reports" Russia has opened a criminal case against a US citizen on suspicion of espionage.
"We're looking into this matter and we'll continue to monitor," State Department deputy spokesman Vedant Patel told reporters on Thursday.
Patel said Russia does not generally abide by obligations to provide timely notification of the detention of US citizens in Russia.
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 nine years in prison on drug charges.
She was exchanged for Viktor Bout, a Russian arms dealer who spent 14 years in jail in the US 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.