South Korea summoned Russia's ambassador, demanding the “immediate withdrawal” of North Korean troops it says are being trained to fight in Ukraine.
About 1,500 North Korean troops, including special forces, have already arrived in Russia, according to Seoul's intelligence agency.
In a meeting with Ambassador Georgiy Zinoviev, South Korean Deputy Foreign Minister Kim Hong-kyun denounced the move and warned that Seoul would “respond with all available measures.”
Mr Zinoviev said he would relay his concerns, but stressed that cooperation between Moscow and Pyongyang was “within the framework of international law”.
It is unclear what cooperation he was referring to. The ambassador did not confirm allegations that North Korea sent troops to fight alongside the Russian military.
Pyongyang has also not commented on the allegations.
South Korea has long accused the North of supplying Russia with weapons for the war against Ukraine, but it says the current situation goes beyond the transfer of military equipment.
Some South Korean media suggest that up to 12,000 North Korean troops are expected to be deployed.
“(This) seriously threatens not only South Korea but also the international community,” Kim said Monday.
Moscow and Pyongyang have stepped up cooperation after their leaders Vladimir Putin and Kim Jong Un signed a security pact in June and pledged their countries would help each other in the event of “aggression” against either. other country.
Last week, Putin introduced a bill aimed at ratifying the pact.
Pyongyang's deployment of troops to fight Russia “would mark a significant escalation” of the conflict, NATO Secretary General Mark Rutte said on Monday.
The United States and Japan have also condemned the strengthening of military ties between North Korea and Russia.
Some defense experts told the Korean BBC that North Korea's involvement could complicate the war.
“North Korea's involvement could open the door to greater international participation in the conflict, potentially attracting more countries,” said Moon Seong-mok of the Korea Institute of National Strategy.
“The international community will likely increase sanctions and pressure on Russia and North Korea, but it remains to be seen whether North Korea's involvement will actually benefit either country,” said Dr. Moon.
But others believe that Russian military units will have difficulty integrating North Korean troops on their front lines.
Aside from the language barrier, the North Korean military has no recent combat experience, they said.
Valeriy Ryabykh, editor-in-chief of the Ukrainian publication Defense Express, said North Korean soldiers could be asked to guard sections of the Russian-Ukrainian border, which would free up Russian units to fight elsewhere.
“I exclude the possibility that these units will immediately appear on the front line,” he said.
Additional reporting by Sangmi Han, Jake Kwon and Hosu Lee in Seoul
