Russia's domestic security agency said Monday that it has arrested a US citizen on espionage charges.

Paul Whelan was detained in Moscow on Friday.

The agency said in Monday's statement that he was caught "during an espionage operation," but didn't give any details.

The US Embassy in Moscow had no immediate comment.

The State Department said it's "aware of the detention of a US citizen by Russian authorities."

Russia's domestic security agency said Monday that it has arrested a US citizen on espionage charges.

The Federal Security Service, or FSB, the top KGB successor agency, said that Paul Whelan was detained in Moscow on Friday. The agency said in Monday's statement that he was caught "during an espionage operation," but didn't give any details.

Spying charges carry a prison sentence of up to 20 years in Russia.

The Russian Foreign Ministry said the US Embassy was duly notified about the arrest, but wouldn't give any further information.

The US Embassy in Moscow had no immediate comment.

Meanwhile, the State Department said it's "aware of the detention of a US citizen by Russian authorities."

"We have been formally notified of the detention by the Ministry of Foreign Affairs. Russia's obligations under the Vienna Convention require them to provide consular access. We have requested this access and expect Russian authorities to provide it," a State Department spokesperson told INSIDER.

The spokesperson added, "Due to privacy considerations, we have no additional information to provide at this time."

The arrest comes as Russia-US ties have sunk to post-Cold War lows over the Ukrainian crisis, the war in Syria and the allegations of Russian meddling in the 2016 US presidential election.

Alexander Mikhailov, a retired FSB officer, said the arrest reflected the high efficiency of Russian counterintelligence.

"The service wouldn't have made this information public unless it had solid evidence," he told the RIA Novosti news agency.