South Korea summoned Russia's ambassador to Seoul Friday to protest a defence deal signed by President Vladimir Putin and North Korea's Kim Jong Un during a visit to Pyongyang this week.
Seoul "strongly urged Russia to immediately stop military cooperation with North Korea and comply with (UN) Security Council resolutions," the foreign ministry said in a statement.
Summoning Russia's Ambassador Georgy Zinoviev came two days after Putin and Kim signed a "comprehensive strategic partnership" agreement, which includes a pledge to come to each other's aid if attacked.
Putin also said this week that Moscow did not rule out military and technical cooperation with the North, or even sending weapons -- all of which would violate rafts of UN sanctions against Pyongyang.
Seoul urged Russia to live up to its responsiblities as a permanent member of the Security Council, the foreign ministry said.
"Violating Security Council resolutions and supporting North Korea, will harm our security and inevitably have a negative impact on Korea-Russia relations," first Vice Minister Kim Hong-kyun said, according to the statement.
North Korea has been under UN sanctions since 2006 over its banned nuclear programme. The measures were intially supported by Russia, but Putin said in Pyongyang that the sanctions should now be reviewed.
Moscow and Pyongyang have been allies since North Korea's founding after World War II and have drawn even closer since Russia's invasion of Ukraine in 2022 isolated Putin on the global stage.
Seoul, which is a major weapons exporter, said this week that it would "reconsider" a longstanding policy that bars it from supplying arms directly to Ukraine, following the North's mutual defence agreement with Russia.
Putin warned South Korea Thursday that this would be a "big mistake".