Seoul, South Korea - South Korea summoned the Russian ambassador to Seoul Monday to criticize North Korea's decision to send thousands of soldiers to support Moscow's war on Ukraine.

About 1,500 North Korean special forces soldiers are already in Russia acclimatizing, likely to head to the front lines soon, Seoul's spy agency said Friday, with additional troops set to depart soon, Pyongyang's first such deployment overseas.

South Korea, which has long claimed the nuclear-armed North is supplying Russia with weaponry for use in Ukraine, has expressed alarm over the deployment, which comes after Kim Jong-un and Russian President Vladimir Putin signed a military deal in June.

Vice foreign minister Kim Hong-kyun expressed Seoul's "grave concerns regarding North Korea's recent dispatch of troops to Russia and strongly urged the immediate withdrawal of North Korean forces and the cessation of related cooperation," the ministry said in a statement.

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Kim told the Russian ambassador to South Korea, Georgiy Zinoviev, that North Korea supplying Russia with weaponry and troops for the war in Ukraine "poses a significant security threat not only to South Korea but to the international community."

He also "emphasized that such actions violate multiple UN Security Council resolutions and the UN Charter."

On Friday, Seoul's National Intelligence Service (NIS) released detailed satellite images it said showed the first deployment of elite North Korean soldiers being moved by Russian military vessels to Vladivostok.

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 first contingent of troops – which South Korean media said were from an elite unit under North Korea's Special Operations Forces, also known as the Storm Corps – are currently stationed in military bases across Russia's Far East.

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The special forces "are expected to be deployed to the front lines as soon as they complete acclimatization training," according to the NIS.

The NIS also said Friday that 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.