A South Korean man has been detained in Russia on suspicion of espionage, local media has reported.
South Korean officials have confirmed the arrest and say its diplomats have engaged in talks for his return.
Baek Won-soon was arrested in the far-eastern city of Vladivostok "at the start of the year", state agency Tass reported, and is now in a Moscow jail.
He is believed to be the first South Korean detained in decades in Russia on spying charges.
Little is known about his case, with South Korea's foreign ministry saying "the details" were under investigation and "it is difficult to comment". Its representatives were attempting provide consular support.
But Russian state media Tass quoted an unnamed official in their report saying Mr Baek is alleged to have passed on information "constituting state secrets to foreign intelligence services."
An anonymous source in Russia's far eastern region, who claims to know Mr Baek, told the OceanNewsUK that Mr Baek had been supporting North Korean labourers working in Russia and helping them escape.
South Korean media have reported that Seoul was only officially informed by Russian authorities of the arrest last month.
Yonhap, a South Korean outlet, is reporting that Mr Baek was arrested by Russia's Federal Security Service in January after arriving in Vladivostok from China.
The outlet said he is a religious worker and he had been accompanied by his wife, who was also detained but then later released. She is believed to be back in South Korea, the outlet reported.
Mr Baek's detention is the latest in a series of arrests of foreigners made by Russian authorities since the war in Ukraine began two years ago. Moscow has been accused of arresting other countries' nationals to use as a political bargaining chip and for prisoner exchanges.
Among the cases are American man and Wall Street Journal reporter Evan Gershkovich who was detained last March on espionage charges and a Russian-US journalist Alsu Kurmsasheva who was accused of spreading "false information" about the Russian military.
Tass reported that Mr Baek is being held in the same prison as Mr Gershkovich - the Lefortovo Prison. A court on Monday ruled for his detention to be extended to June, the Russian news agency reported.
South Korea has with its Western allies condemned Russia's invasion of Ukraine and committed to a sanctions regime against Moscow. At the same time, Russia and North Korea have cultivated closer ties - with Vladimir Putin and Kim Jong un meeting in high-profile visits last year which analysts believe spawned military technology deals.
A number of meetings between Russian and North Korean officials also took place in Vladivostok last year.
