South Korea summoned the Russian ambassador on Monday to criticise Pyongyang's decision to send thousands of soldiers to support Moscow's war in Ukraine, the foreign ministry said, calling for their immediate withdrawal.
In Pyongyang's first such deployment overseas, about 1,500 North Korean special forces soldiers are already in Russia acclimatising and likely to head to the front lines after, Seoul's spy agency said Friday, with additional troops set to depart soon.
South Korea has long claimed the nuclear-armed North is supplying Russia with weapons for use in Ukraine, while leader Kim Jong Un and Russian President Vladimir Putin signed a military deal in June.
Seoul expressed its "grave concerns regarding North Korea's recent dispatch of troops to Russia and strongly urged the immediate withdrawal of North Korean forces", vice foreign minister Kim Hong-kyun told Russian ambassador Georgiy Zinoviev.
Seoul's spy agency released detailed satellite images showing the first batch of 1,500 North Korean special forces from the elite "Storm Corps" had arrived in Vladivostok on Russian military vessels.
Any military cooperation between the two countries violates multiple Security Council resolutions, Kim said.
Russian ambassador Zinovyev "stressed that cooperation between Russia and North Korea... is not directed against the interests of South Korea's security", the embassy said in a statement.
The Kremlin's spokesman, Dmitry Peskov, also said Monday that Moscow will "continue developing this cooperation further".
"North Korea is our close neighbour and partner and we develop relations in all areas and it's our sovereign right," he told journalists in Moscow, while declining to comment on whether Russia is using North Korean troops.
Also Monday, South Korea's President Yoon Suk Yeol spoke to NATO chief Mark Rutte, urging the alliance to take "concrete countermeasures" against growing Russia-North Korea cooperation.
NATO, which has not yet confirmed the North Korean troop deployments, said that it "would mark a significant escalation" in the conflict, Rutte said on X.
British foreign minister David Lammy, who was in Seoul on Monday, called Russia's actions "reckless and illegal" and added that London would work with Seoul to respond, according to Yoon's office.
North Koreans in Ukraine?
"South Korea's protest to Russia will not change anything regarding Moscow's military cooperation with the North," said Cheong Seong-chang, director of the Korean peninsula strategy at Sejong Institute.
In return for sending soldiers to help Russia in Ukraine, "Kim Jong Un is aiming to acquire military technologies, ranging from surveillance satellites to submarines", he said.
The North Korean soldiers will likely soon be on Ukraine's frontlines, he said, adding "it remains to be seen how much impact they will have in the course of the conflict".
Seoul's spy agency said that between October 8 and 13, "North Korea transported its special forces to Russia via a Russian Navy transport ship, confirming the start of North Korea's military participation" in Moscow's war in Ukraine.
The special forces now stationed in Russian bases in the Far East "are expected to be deployed to the front lines (of the Ukraine conflict) as soon as they complete acclimatization training", according to the NIS.
The NIS also said Friday the North had "provided Russia with more than 13,000 containers' worth of artillery shells, missiles, anti-tank rockets, and other lethal weapons" since last August.
Pyongyang and Moscow have been allies since North Korea's founding after World War II, and have drawn even closer after Russia's 2022 invasion of Ukraine.
Last week, Ukrainian President Volodymyr Zelensky flagged intelligence reports saying North Korea was training 10,000 soldiers to support Russia, saying Moscow was relying on the North to make up for its substantial losses.
South Korea, one of the world's top 10 weapons exporters, has long resisted calls from its allies including Washington to supply Kyiv with weapons -- pointing to long-standing domestic policy which bars Seoul from selling weapons into active conflict zones.
It has however sold billions of dollars of tanks, howitzers, attack aircraft, and rocket launchers to Kyiv's key ally Poland, and Polish President Andrzej Duda is set to visit Seoul from Tuesday this week.