Russia’s domestic security agency said Monday that it has arrested a U.S. 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 U.S. Embassy was duly notified about the arrest, but wouldn’t give any further information.

The U.S. Embassy in Moscow had no immediate comment.

The arrest comes as Russia-U.S. 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 U.S. presidential election.

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

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

Story by The Associated Press