South Korea on Monday demanded the immediate pullout of North Korean troops allegedly deployed in Russia as it summoned the Russian ambassador to protest deepening military cooperation between Pyongyang and Moscow.

South Korea’s spy agency said Friday it had confirmed that North Korea sent 1,500 special operation forces to Russia earlier this month to support Moscow’s war against Ukraine. Ukrainian President Volodymyr Zelenskyy earlier said his government had intelligence that 10,000 North Korea soldiers were being prepared to join invading Russian forces.

During a meeting with Russian Ambassador Georgy Zinoviev, Vice South Korean Foreign Minister Kim Hong Kyun “condemned in the strongest terms” North Korea’s troop dispatch that he said poses “a grave security threat” to South Korea and the international community, the South Korean Foreign Ministry said in a statement.

Kim said that South Korea in collaboration with the international community will mobilise all available means to deal with an act that threatens its vital national security interests. Zinoviev replied he would relay South Korea’s position to his home government, according to the South Korean statement.

The Russian Embassy quote Zinoviev as saying that the Russian-North Korean cooperation is conducted in accordance with international laws and is not aimed against the security interests of South Korea.

The US and NATO haven’t confirmed that North Korean troops were sent to Russia. But the reports of their presence have already stoked concerns in South Korea that Russia might provide North Korea with sophisticated technologies that can sharply enhance its nuclear and missile programs in return for the troops.

North Korea’s advancing nuclear arsenal is a major security threat to South Korea. North Korean leader Kim Jong Un recently took steps to permanently terminate all relations with South Korea and threatened to use nuclear weapons preemptively. Some observers say South Korea will likely consider supplying weapons to Ukraine if Russian transfers of high-tech nuclear and missile technologies to North Korea are verified.

South Korea has joined US-led sanctions against Moscow over its invasion of Ukraine in February 2022. But it hasn’t directly provided arms to Kyiv, citing a longstanding policy of not supplying weapons to countries actively engaged in conflicts.

Russia has earlier denied using North Korean troops in its war with Ukraine. North Korea’s state media hasn’t commented on the matter.

North Korea’s troop deployment to Russia would be its first participation in a major war since the end of the 1950-53 Korean War. Many experts question how much North Korean troops would help Russia on the battlefield, citing their lack of combat experience.

Cooperation between North Korea and Russia has flourished over the past two years. The US, South Korea and their partners have accused North Korea of supplying  conventional arms to Russia in return for economic and military assistance. In June, Kim and Russian President Vladimir Putin signed a pact stipulating mutual military assistance if either country is attacked.