BRUSSELS: If North Korea sent soldiers to support Russia in Ukraine, that would be "a significant escalation", NATO secretary general Mark Rutte warned on Monday.

Rutte said he spoke to South Korean President Yoon Suk Yeol about the Alliance's "close partnership with Seoul, defence industrial cooperation, and the interconnected security of the Euro-Atlantic and Indo-Pacific".

"North Korea sending troops to fight alongside Russia in Ukraine would mark a significant escalation," he added in a post on X.

Seoul's spy agency said on Friday that North Korea had decided to send a "large-scale" troop deployment to support Moscow's war in Ukraine, with 1,500 special forces already in Russia's Far East and undergoing training.

The agency estimated the North could send around 12,000 soldiers in total. Rutte had said on Friday that NATO could not confirm South Korean intelligence "at this moment" but added: "This, of course, might change."

Pyongyang and Moscow have been allies since North Korea's founding after World War II.

They have drawn even closer since Russia's 2022 invasion of Ukraine, with Seoul and Washington claiming that Kim Jong Un has been sending weapons for use in Ukraine.

Meanwhile, 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 programme, 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.

 - AFP