SEOUL, March 12, 2024 (BSS/AFP) - Seoul is providing "necessary consular assistance" to a South Korean national who has been detained in Russia, Seoul's foreign ministry said Tuesday, reportedly on espionage charges.

Russia and North Korea have recently bolstered ties, including -- the South and United States say -- Pyongyang shipping arms to Moscow for use in Ukraine.

South Korea has been designated an "unfriendly country" by Moscow, after Seoul imposed sanctions on Russia over its war in Ukraine.

Seoul's foreign ministry on Tuesday declined to give details on the South Korean's detention, citing an ongoing investigation.

"Upon learning of the arrest, the local diplomatic mission has been providing necessary consular assistance," it said.

Russia's state-run TASS news agency first reported that a South Korean man had been detained earlier this year on suspicion of espionage and was being held in a Moscow prison.

TASS, which said the case was "top secret", identified the suspect as Baek Won-soon and said he had been picked up in Russia's far-eastern city of Vladivostok.

"According to a TASS source in law enforcement agencies, Baek Won Soon passed state secret information to foreign intelligence services," the agency said.

It is the first time Russia has arrested a South Korean citizen for criminal espionage, according to TASS.

Baek was transported from Vladivostok to Moscow for further investigation and is being held in the capital's notorious Lefortovo prison, the Russian news agency added.

The prison, known for keeping detainees in near-total solitude, also currently houses US reporter Evan Gershkovich, held on spying charges that he denies.

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

North Korean leader Kim Jong Un visited his Russian counterpart Vladimir Putin last year, Pyongyang has been drawing ever closer to Moscow.

During his trip to Russia, Kim declared that bilateral ties with Moscow were his country's "number one priority".

An analyst told AFP that North Korea could have been involved in the arrest, given the close ties between Moscow and Pyongyang.

"It cannot be ruled out that Russian intelligence agencies received intel from North Korean spies about the South Korean suspect and arrested him," said Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies.

"It is also likely to be retaliation against the South Korean government for indirectly providing weapons to Ukraine."