South Korea's Foreign Ministry summoned the Russian ambassador to protest North Korea's alleged deployment of troops to Ukraine, according to a report by Yonhap on Monday.
Georgy Zinoviev, the top Russian envoy to Seoul, told Yonhap that he met with South Korea's first vice foreign minister, Kim Hong-kyun, earlier on the day, according to the news agency.
South Korea's foreign ministry did not immediately respond to a request for comment.
South Korea's spy agency said last week that North Korea had shipped 1,500 special forces troops to Russia's Far East for training and acclimatizing at local military bases and will likely be deployed for combat in the war in Ukraine.
The White House National Security Council could not confirm reports that North Korean troops were fighting for Russia, a spokesperson said on Friday but if true, "this would mark a dangerous development in Russia’s war against Ukraine."
Zelenskyy demands response
Earlier Sunday, Ukrainian President Volodymyr Zelenskyy said that the military cooperation between Russia and North Korea marks a new escalation in Moscow's war of aggression against Ukraine.
"We must respond and counteract. We must not allow evil to increase further," Zelenskyy said in his evening video message.
"If the world remains silent now and we are confronted with North Korean soldiers on the front line just as regularly as we defend ourselves against drones, this will not benefit anyone in the world and will only prolong this war."
Moscow has for days been denying reports from South Korea and Ukraine, which also rely on satellite images, about the alleged deployment of up to 12,000 North Korean soldiers to Russia.
Neither NATO nor U.S. officials have confirmed that North Korea is sending soldiers to reinforce Russia.
Russia's and North Korea's cooperation includes close military ties and mutual assistance in the event of external attacks.
Such soldiers could be deployed in the defence of the Russian region of Kursk, which Ukraine has invaded. Since early August, Kyiv has occupied dozens of places in the Kursk area.
Zelenskyy referred to the cooperation as a "malicious alliance" and "dangerous cooperation" between Russia and North Korea. He says the level of instability and threat could increase significantly once North Korea learns the tactics of modern warfare.
He urged Ukraine's allies to have a response ready, as another state becoming a participant in the war poses a new threat. Up until now, Putin has primarily used neighboring Belarus as a staging area for his war.
Zelenskyy stated that the war must be ended justly and as quickly as possible. Ukraine has been defending itself against the Russian war of aggression with Western military aid for almost 1,000 days.