The man, named as Baek Won-soon, was transferred from Vladivostok to Moscow and is accused of handing over classified information to foreign intelligence agencies, Russian state news agency Tass reported.
Baek, 53, was described as a “deeply religious” person who was also registered as the founder of a travel company based in Vladivostok, according to Tass.
It was the first time a South Korean has been detained in Russia for alleged espionage, Tass said.
South Korean television network JTBC said Baek had travelled to Vladivostok from China earlier this year to carry out missionary work for North Korean workers in Russia, citing an unidentified acquaintance.
He was also involved in helping North Koreans to defect, Yonhap news agency quoted an unnamed acquaintance as saying.
South Korea’s foreign ministry said its consulate had been providing assistance since it became aware of the arrest. It declined to give more details as the matter was currently under investigation.
The South Korean government is communicating with Russia for the safe return of its national, a foreign ministry spokesperson told reporters on Tuesday.
U.S. and South Korean officials have raised concerns that Russia has accepted new groups of North Korean workers in defiance of a U.N. resolution amid a blossoming of ties between Moscow and Pyongyang.
A 2017 U.N. Security Council resolution required countries to expel North Korean workers by 2019 on the grounds that their labor was exploited to earn foreign currency for North Korea’s banned nuclear and ballistic missile programs. But thousands reportedly remain in China and Russia.
North Korean construction workers and loggers still remain in Russia, holding student or tourism visas there, a South Korean government report said last year.
Ties between Russia and South Korea have been increasingly strained by Moscow’s growing relations with North Korea amid allegations that Pyongyang has supplied munitions for the war in Ukraine.
South Korea, a U.S. ally, has denounced Russia’s invasion and supplied economic and humanitarian aid to Kyiv, but has so far stopped short of sending weapons.
Since the Ukraine war, South Korea has had a special travel advisory urging its citizens not to travel to Russia.