Baek Won-soon was detained in Vladivostok earlier this year – the latest foreign national to be imprisoned in Russia.

Citing the authorities, the Russian state-run TASS news agency identified the man as Baek Won-soon and said he had been detained in the city of Vladivostok “at the start of the year”, before being transferred to Moscow for “investigative actions” at the end of last month.

Baek, whose case has been classified as “top secret”, is being held in Lefortovo Prison, where a court on Monday ordered his detention to be extended until June 15, TASS said.

The agency cited an unnamed law enforcement official as saying Baek had passed on information “constituting state secrets to foreign intelligence services.” No further details were made public.

South Korea’s Ministry of Foreign Affairs said in a statement its consulate had been providing assistance since it became aware that Baek had been detained. It declined to give more details on the matter citing the ongoing investigation.

The Yonhap news agency’s Korean service said Baek was a missionary who had been involved in rescuing North Korean defectors and providing humanitarian aid. He was detained in January a few days after arriving in Vladivostok by land from China, the agency added.

The incident marks the first time a South Korean has been detained in Russia on spying charges.

Russia labelled South Korea an “unfriendly” country in 2022 because of its support for Western sanctions against Moscow over its invasion of Ukraine.

Russia has also deepened relations with North Korea after leader Kim Jong Un travelled to Russia last September and met Russian President Vladimir Putin.

The United States and others have accused North Korea of providing weapons to Russia for use in its war in Ukraine in exchange for technological know-how to advance Pyongyang’s military modernisation programme.

Both countries have denied the allegations.

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

US journalist Evan Gershkovich was detained for alleged espionage in March 2023 and is also being held at Lefortovo prison, which is notorious for its harsh conditions and keeping detainees in near-total isolation. His detention has been extended until the end of March with court proceedings held behind closed doors.

In October, Russian-US journalist Alsu Kurmasheva was detained for failing to register as a foreign agent and later charged with spreading “false information” about the Russian military. Her detention has been extended until April.

Espionage carries a maximum jail term of 20 years in Russia.

Gershkovich and Kurmasheva both deny the charges against them.