MOSCOW (AP) — 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 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 U.S. State Department said it was “aware of the detention of a U.S. citizen by Russian authorities” and had received formal notification from the Russian Foreign Ministry. It said it was pushing for consular access to the detained American.

“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,” the department said.

The State Department did not confirm the name of the detained citizen or provide any information about the case, citing “privacy considerations.”

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

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.