A South Korean citizen arrested in Russia earlier this year on suspicion of spying is to remain in custody until mid-June, Russia's state news agency Tass said Monday.

The news agency said it is the first time that a South Korean has been arrested for espionage in Russia.

Tass said the man, identified by the agency as Baek Won-soon, was detained “at the start of the year” in the far eastern city of Vladivostok and was taken to Moscow at the end of February. He is being held in Lefortovo Prison, where a court on Monday ordered his detention extended until June 15, Tass said.

Tass cited an unnamed law enforcement official as saying the suspect passed on information “constituting state secrets to foreign intelligence services." The case has been labeled “top secret” and no further details have been made public.

South Korea’s Foreign Ministry said Tuesday it has been providing necessary consular service to the man since finding out about his arrest. But the ministry declined to provide further details saying the case was under investigation.

Over the past year, Russia has detained multiple foreign nationals and accused them of committing various offenses.

Last March, Russia detained Evan Gershkovich, a Wall Street Journal reporter who was on a business trip to Russia, on espionage charges. In October, Russian-U.S. journalist Alsu Kurmasheva was detained for failing to register as a foreign agent and later charged with spreading “false information” about the Russian military.

Last month, Tass reported a German man had been detained and accused of smuggling drugs. Also in February, Russian state news agencies reported a dual U.S.-Russian woman identified as Ksenia Khavana was arrested on charges of committing treason.

Russia has previously been accused of targeting foreign citizens to use them as bargaining chips to secure the release of Russian prisoners abroad. The Russian government has denied the accusations.

In a recent interview, Russian President Vladimir Putin appeared to suggest that in exchange for Gershkovich, Russia is seeking the return of Vadim Krasikov, who is serving a life sentence in Germany after being convicted of the 2019 killing of Zelimkhan “Tornike” Khangoshvili, a 40-year-old Georgian citizen of Chechen ethnicity.

Gershkovich is also held in Lefortovo Prison, notorious for its harsh conditions. The prison dates from the czarist era and has been a terrifying symbol of repression since Soviet times. (AP)