SEOUL, Oct 21, 2024 (BSS/AFP) - South Korea summoned the Russian ambassador 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.

About 1,500 North Korean special forces soldiers are already in Russia acclimatising and 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 has long claimed the nuclear-armed North is supplying Russia with weapons for use in Ukraine, and Seoul expressed alarm over the troop deployment, which comes after 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.

The move "poses a significant security threat not only to South Korea but to the international community", Kim said.

Both North Korea and Russia are under rafts of UN sanctions -- Kim Jong Un for his weapons program, and Moscow for the war in Ukraine.

The two countries' military cooperation violated Security Council resolutions, Kim Hong-kyun 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.

NATO, which has not yet confirmed the North Korean troop deployments, said that it "would mark a significant escalation" in the conflict, chief Mark Rutte said Monday on X.

- 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 acclimatisation 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.

Pyongyang and Moscow have been allies since North Korea's founding after World War II, and have drawn even closer since 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 in its fight against Kyiv, and said Moscow was relying on the North to make up for its substantial losses.

Ukrainian media reported earlier this month that six North Korean military officers were killed in a Ukrainian missile attack on Russian-occupied territory near Donetsk -- which South Korea's defence minister said at the time was "highly likely" to be true.

South Korea, one of the world's top ten 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.